Labile Soil Research Articles

Effects of Warming and Fertilization on Soil Organic Carbon and Its Labile Components in Rice-wheat Rotation

Farmland is the important soil carbon pool of terrestrial ecosystems and organic nutrient pool for crop growth. To clarify the impact of climate warming on the soil carbon pool, this study analyzed the effects of warming and fertilization on soil organic carbon and its labile components under rice-wheat rotation using a free-air temperature increase system. The variation in soil carbon pool management index (CPMI) was also evaluated. The results showed that the combined effects of warming and fertilization on soil organic carbon content and labile organic carbon components were insignificant. Warming increased the soil organic carbon (SOC) content, and the differences between warming and the ambient control in total organic carbon (TOC) and recalcitrant organic carbon (ROC) reached a statistically significant level. Compared with those under the ambient control, the contents of TOC, ROC, and labile organic carbon (LOC) subjected to warming increased by 7.72%, 7.42%, and 10.11%, respectively. The increased microbial biomass carbon (MBC) content (20.4%) and decreased particulate organic carbon (POC) content (36.51%) may have been the main reason for the variation in SOC. Warming showed no significant effect on soil dissolved organic carbon (DOC) content, whereas it markedly reduced its soluble microbial by-product components (41.89%). The results also showed that fertilization had no significant effect on soil TOC, ROC, and LOC, but it notably reduced the contents of DOC and POC and increased the MBC content. Compared with those under the control without fertilization, the contents of DOC and POC subjected to fertilization decreased by 35.44% and 28.33%, respectively, and the MBC content increased by 33.38%. Additionally, fertilization tended to increase the anthropogenic humus component (5.13%) and soluble microbial by-product component (29.41%) in dissolved organic matter and reduce the terrestrial humus component (13.33%). Warming and fertilization both tended to improve soil CPMI. Affected by SOC and LOC, the increase in soil carbon pool index and soil lability index were the main reason for the increase in soil CPMI under warming and fertilization, respectively. Overall, the results revealed that climate warming can affect the soil carbon pool by changing soil labile carbon components, which are not affected by fertilization.

Amendments of waste ochre from former coal mines can potentially be used to increase soil carbon persistence

There is an increasingly urgent need to remove CO2 from the atmosphere. Increasing C storage in soils has been discussed as a possible method of achieving this. Regardless of whether increasing soil C would result in reduced atmospheric C or not, increasing the amount of organic carbon (OC) stored as organic matter in soils is widely regarded as leading to agronomic benefits such as increased water retention and resistance to erosion. Fe oxides adsorb OC in soil, protecting it from degradation. In this study we investigated the use of waste ochre (Fe oxy-hydroxide) that precipitates from the drainage of former coal mines as a soil amendment to increase C storage in soils via reduced C lability. In preliminary batch experiments, ochres reduced the release of OC from soils into solution. In plant growth experiments with wheat (var. Skyfall, RAGT) 5 wt% amendments of ochre to soils significantly reduced the concentration of hot water extractable OC by approximately 16% and OC lost from the soils in leachate by approximately 43%. Above ground plant biomass was significantly reduced by approximately 50% in the amended soils. There was no evidence for increased uptake of potentially toxic elements in the plants from the ochre-amended soils compared to the study controls but Olsen P extractable phosphate was decreased by the ochre amendments. In subsequent experiments with agronomically realistic additions of KH2PO4 there was no difference in plant biomass between the study controls and the ochre-treated soils suggesting that reduced plant growth was due to reduced P availability; C lability was still significantly reduced. Furthermore, the ochre amendments helped retain th added phosphate in the soil. These results indicate that Fe oxide amendments to soil may be a viable way of changing soil chemistry in order to increase the amount of OC retained in soils. Further experiments investigating the impacts of the ochre amendments on greenhouse gas emissions and soil biology are required followed by field trials where the impact of more variable soil moisture and temperature effects can be assessed and crops grown to harvest. A full Life Cycle Analysis could then be performed. Given the relatively limited quantities of former coal mine ochres available in the UK and the carbon footprint of transporting materials prior to adding them to soils, similar experiments to those reported here, taking into account chemical and mineralogical variations, are warranted to determine the global stock pile of Fe-rich wastes that could potentially be used to reduce C lability in soils.

Short-term response of soil aggregate stability and labile carbon to contour tillage, diversion terrace, grassed waterway, and tile drainage implementation

Soil degradation has been of great concern for New Brunswick's potato farmers, especially on sloped land and shallow soils. In this study, we evaluated the initial response of labile soil carbon (C) fractions (permanganate oxidizable C (POXC) and particulate organic C (POC)) and aggregate stability to two integrated best management practices (BMPIs) composed of the following individual practices: diversion terraces (DT), grassed waterways (GW), and contour tillage (CT) (i.e., DTGW) and DT, GW, CT, and tile drainage (TD) (i.e., DTGW + TD), relative to CT that served as a control. The more water was regulated in the field, the greater the increase in labile C; where DTGW and DTGW + TD gained 19.8% and 50.6% of POXC, respectively, while CT lost 11.2% of POXC. There was an increase in mineral associated organic matter C in the terraced BMPIs, despite the high amount of tillage events that took place during potato cultivation. Two BMPIs had no effect on aggregate stability, most likely due to the short duration of this initial monitoring study that spanned just over two growing seasons. Even though there were no improvements in soil structure, our findings showed that some stabilization of soil C is possible even during the initial two seasons following BMPI implementation.

Modification of the RZWQM2-P model to simulate labile and total phosphorus in an irrigated and manure-amended cropland soil

With the expansion of the dairy industry, phosphorus (P)-enriched dairy manure has increasingly been used to replace chemical fertilizer to meet crop nutrient demand. This practice could lead to excessive total P accumulation in the soil and increase the risk of P pollution in the environment. The newly-developed RZWQM2-P model uses the soil P pool structure from the EPIC model, which is not sensitive to total soil P. Therefore, we modified the P module in RZWQM2-P to improve its capability in simulating total soil P. We subsequently assessed the ability of the modified model to simulate labile soil P, total soil P, plant P uptake, and crop yield using a dataset collected from an irrigated field treated with dairy manure and inorganic fertilizer at eight rates under a repeating wheat-potato-barley-sugar beet rotation. The results suggested that the modified RZWQM2-P model satisfactorily simulated field-measured annual total soil P, plant P uptake, and crop yield. Labile soil P was simulated less accurately, but the results were acceptable as the model responded well to P treatments. We simulated the long-term soil P dynamics under three P-application scenarios. Long-term simulation results showed that it took 14 years for the labile soil P level to return to the initial level after eight years of manure-P applications at a rate of 65.5 kg P/ha year−1. The modified RZWQM2-P model can be used to simulate total soil P and labile soil P contents and to assess P management practices in irrigated cropland amended with manure.

Effects of warming on the stoichiometry of soil microbial biomass and extracellular enzymes in an alpine shrubland

The alpine shrublands on the Qinghai-Tibetan Plateau are experiencing significant warming. Revealing the effects of warming on the soil microbial metabolism will provide insights into the dynamics of soil carbon (C) and nutrient cycling in these alpine ecosystems. A five-year warming experiment (+1.3 °C, using open-top chambers) was conducted in a Sibiraea angustata-dominated alpine shrubland on the eastern Qinghai-Tibetan Plateau. We examined the effects of warming on the stoichiometry of topsoil (0–15 cm) microbial biomass, extracellular enzyme activity and their relationships with soil physicochemical properties. The results showed that the effects of warming on the soil microbial biomass C, nitrogen (N) and phosphorus (P) varied with the growing season. Warming increased the activity levels of C-acquiring enzymes (β-glucosidase, BG) by 7.5 %-15.1 % throughout the growing season (P < 0.01) and the activity of polyphenol oxidase (PPO) by 6.5 % during the early growing season (P < 0.05), indicating that warming would strengthen microbial C limitation and increase degradation of soil labile and recalcitrant C components. Warming increased the activity of P-acquiring enzyme (acid phosphatase, AP) by 14.6 % (P < 0.01) and triggered microbial P limitation during the late growing season, which might be a result of an increasing alpine shrub P uptake. Warming did not affect the total activity levels of N-acquiring enzymes throughout the growing season (P > 0.05), mainly due to higher soil N availability. Warming did not affect the stoichiometry of soil extracellular enzymes throughout the growing season (P > 0.05), except for a decreasing in the enzyme N:P ratio during the early growing season (P < 0.05). Moreover, the microclimate (air and soil temperature), available soil nutrients (soil dissolved organic C and available P) and soil pH accounted for most of the variation in soil microbial biomass, extracellular enzyme activity and their stoichiometry in response to warming. Our results suggest that warming may trigger and strengthen soil microbial C and P limitation during the growing season, accelerating soil C loss and nutrient cycling in alpine shrublands on the Qinghai-Tibetan Plateau.

Soil aggregation and associated organic matter under management systems in sandy-textured soils, subtropical region of Brazil.

Increasing the diversity of plant species in agricultural production areas favors the maintenance or improvement of soil quality, particularly for soils with a sandy texture. This beneficial effect is related to the formation of aggregates of different origins. This study aimed to (i) verify whether soil use and management affect the proportion of biogenic (Bio) and physicogenic (Phy) aggregates and (ii) verify whether biogenic aggregation is more likely to lead to soil improvement than physicogenic aggregation. Three management systems were evaluated (permanent pasture, PP; no-tillage system, NT; and no-tillage + Brachiaria system, NT + B) as well as a reference area (Atlantic Forest biome vegetation, NF). According to their origin or formation pathway, the aggregates were separated, identified, and classified as Bio (formed by biological processes) and Phy (resulting from chemical and physical actions). The differentiation between Bio and Phy aggregates was performed based on the visualization of morphological features, such as shape, size, presence of roots, porosity, and subunit arrangements, and junctions. Only the PP area was able to promote greater aggregate formation of biological origin, with greater amounts of Bio aggregates. The highest total organic carbon (TOC) contents and the least negative δ13C values were also quantified in the aggregates of the PP area. The NT + B system provided an increase in the TOC content of its aggregates in comparison with aggregates in the NT and NF areas. Among the formation pathways, the Bio aggregates had the highest TOC and soil organic matter fractions contents and the most negative δ13C values. Perennial forage grasses vegetation was more important than the plant species diversity in favoring Bio aggregate formation. The beneficial effect of Brachiaria can be observed when incorporated as part of intercropping with corn in grain production systems. The biogenic aggregates favored the concentration of more labile soil organic matter fractions. The results of this study can provide important theoretical information for future studies focused on the combination of different plant species in agricultural food production areas on sandy-textured soils.

Dispersed ice of permafrost peatlands represents an important source of labile carboxylic acids, nutrients and metals

Thawing of frozen organic and mineral soils and liberation of organic carbon (OC), macro- and micro-nutrients and trace elements from pore ice in high latitude regions represent a potentially important but poorly quantified retroactive linkage to climate warming. This is especially true for permafrost peatlands, occupying a sizable proportion of all permafrost territories and presenting a large and highly vulnerable stock of soil OC which can be subjected to fast thawing at currently circum-zero temperatures. The conventional method of assessing the labile water-soluble fraction of permafrost soils is aqueous extraction from dried soil. However, this technique does not allow collecting native ice present in soil pores and is therefore likely to underestimate or overestimate the pool of labile soil C and nutrients. Here, we present results of direct pore ice analyses performed on native peat cores from the western Siberia Lowland in comparison to the water extraction (10 and 100 gdry peat L-1) of soluble components from the same peat subjected to freeze drying. Aqueous leachates of permafrost peat from both thawed (0–45 cm) and frozen (45–130 cm) layers yielded high concentrations of DOC, nutrients, carboxylic acids and trace metals, comparable or higher to those in peat porewater and dispersed peat ice. We found strong (a factor of 3 to 30) enrichment in the frozen part of the core (below 45 cm, which is active layer depth) in dissolved OC, many carboxylates (acetate, formate, lactate, butyrate, propionate, pyruvate), inorganic nutrients (Si, P, N) and trace elements (Fe, Al, Mn, Zn, Sr and Ba). The dispersed ice which is present in peat below active layer represents highly labile reservoir of organic and inorganic nutrients which should be considered in permafrost thaw scenarios.

Nano and fine colloids suspended in the soil solution regulate phosphorus desorption and lability in organic fertiliser-amended soils

Mobile colloids impact phosphorus (P) binding and transport in agroecosystems. However, their relationship to P-lability and their relative importance to P-bioavailability is unclear. In soils amended with organic fertilisers, we investigated the effects of nano (NC; 1–20 nm), fine (FC; 20–220 nm), and medium (MC; 220–450 nm) colloids suspended in soil solution on soil P-desorption and lability. The underlying hypothesis is that mobile colloids of different sizes, i.e., NC, FC, and MC, may contribute differently to P-lability in soils enriched with organic fertiliser. NC- and FC-bound Pcoll were positively correlated with P-lability parameters from diffusive gradient in thin films (DGTA-labile P concentration, r ≥ 0.88; and DGTA-effective P concentration, r ≥ 0.87). The corresponding relations with MC-bound Pcoll are weaker (r values of 0.50 and 0.51). NC- and FC-bound Pcoll were also strongly correlated with soil P-resupply (r ≥ 0.64) and desorption (r ≥ 0.79) parameters during DGTA deployment, and the mobility of these colloids was corroborated by electron microscopy of DGTA gels. MC-bound Pcoll was negatively correlated with the solid-to-solution distribution coefficient (r = −0.42), indicating this fraction is unlikely to be the source of P-release from the solid phase after P-depletion from the soil solution. We conclude that NC and FC mainly contribute to regulating soil desorbable-P supply to the soil solution in the DGTA depletion zone (in vitro proxy for plant rhizosphere), and consequently may act as critical conditioners of P-bioavailability, whereas MC tends to form complexes that lead to P-occlusion rather than lability.

In situ degradation of low-density polyethylene film in irrigation maize field: Thickness-dependent effect

Thickness of low-density polyethylene (LDPE) film might determine its mechanical strength, clean production and soil health. Yet, this issue is little understood. In situ aging effects were evaluated in LDPE films with the thickness of 0.006 mm, 0.008 mm, 0.010 mm and 0.015 mm in maize field. The data showed that maximum tensile force (TFmax), maximum tensile strength (TSmax) and elongation at break (EAB) were massively lowered with increasing thickness after aging. The greatest and lowest reduction magnitude of EAB was 27.6 % and 11.2 % in 0.006 mm and 0.015 mm films respectively. Also, the melting point (Tm) and crystallinity (Xc) under Differential Scanning Calorimeter (DSC) tended to decline with the increasing thickness. Moreover, the peak intensity of crystalline regions tended to transfer and concentrate on the amorphous regions, and such tendency became more pronounced in the thin films. Interestingly, there existed a pronounced distinct thickness-dependent effects on soil bulk density (SBD) and soil water-stable aggregate proportion. Thick plastic film mulching increased SBD but reduced the proportion of macroaggregates (mainly referred to 0.015 mm and 0.010 mm). In addition, thick film mulching slightly reduced the levels of soil organic carbon (SOC) and total nitrogen (TN), but significantly promoted the contents of soil labile C and N. Particularly, it significantly promoted above- & under-ground biomass of maize across two growing seasons (p < 0.05). To sum up, thickening LDPE film may act as a promising solution to improve LDPE film residue recycling, while benefiting for higher productivity. However, thick film mulching may cause a certain adverse impact on soil structure, and further investigations would be needed in the future.

5300-Year-old soil carbon is less primed than young soil organic matter.

Soils harbor more than three times as much carbon (C) as the atmosphere, a large fraction of which (stable organic matter) serves as the most important global C reservoir due to its long residence time. Litter and root inputs bring fresh organic matter (FOM) into the soil and accelerate the turnover of stable C pools, and this phenomenon is termed the "priming effect" (PE). Compared with knowledge about labile soil C pools, very little is known about the vulnerability of stable C to priming. Using two soils that substantially differed in age (500 and 5300 years before present) and in the degree of chemical recalcitrance and physical protection of soil organic matter (SOM), we showed that leaf litter amendment primed 264% more organic C from the young SOM than from the old soil with very stable C. Hierarchical partitioning analysis confirmed that SOM stability, reflected mainly by available C and aggregate protection of SOM, is the most important predictor of leaf litter-induced PE. The addition of complex FOM (i.e., leaf litter) caused a higher bacterial oligotroph/copiotroph (K-/r-strategists) ratio, leading to a PE that was 583% and 126% greater than when simple FOM (i.e., glucose) was added to the young and old soils, respectively. This implies that the PE intensity depends on the chemical similarity between the primer (here FOM) and SOM. Nitrogen (N) mining existed when N and simple FOM were added (i.e., Glucose+N), and N addition raised the leaf litter-induced PE in the old soil that had low N availability, which was well explained by the microbial stoichiometry. In conclusion, the PE induced by FOM inputs strongly decreases with increasing SOM stability. However, the contribution of stable SOM to CO2 efflux cannot be disregarded due to its huge pool size.

Cover crops and N fertilization affect soil ammonia volatilization and N2O emission by regulating the soil labile carbon and nitrogen fractions

Introducing cover crops in monocropping systems to improve soil health has been broadly adopted worldwide. However, the impacts of different cover cropping systems on soil emissions and related soil fractions and their regulatory mechanism remain elusive. In this study, four cropping systems, i.e., relay intercropping with Orychophragmus violaceus L./cotton (OvC), Vicia villosa Roth/cotton (VvC), a cover crop mixture (Ov and Vv)/cotton (MSC) and a fallow field–cotton (FFC) system, were combined with four N application rates in a two-rotation cycle (2019–2021) to evaluate the effects on soil greenhouse gases (GHGs) emission, ammonia (NH3) volatilization, and labile carbon (C) and nitrogen (N) fractions and their relationships. Compared with the FFC system, the cumulative NH3 volatilization and nitrous oxide (N2O) emissions of OvC are noticeably reduced 13% and 58%, respectively, and those of VvC are comparable, while those of MSC are markedly increased by 15% and 24%, respectively. N fertilization significantly affects soil NH3 volatilization and N2O emissions, and a higher N rate results in more gaseous N emissions. In addition, NH3 volatilization and N2O emission are positively correlated with the soil microbial biomass N (MBN), dissolved organic N (DON), NH4+-N and NO3--N but negatively correlated with microbial biomass C (MBC) and dissolved organic C (DOC). OvC system decreased gaseous N emission by high soil MBC/MBN ratio, Vv as cover crop indirectly promote soil N2O emission through increased soil MBN, DON and MBC content, while cover crop mixtures increase soil labile C and N fractions and enhance gaseous N emission ultimately. Overall, application of N fertilizer and cover crop mixture both promoted soil NH3 and N2O emissions; the relay intercropping nonlegume cover crop Ov noticeably mitigates N2O and NH3 emissions through an increase in the soil MBC and reduce in the labile N, while introduction of the legume cover crop Vv increases N2O emissions but reduces soil NH3 volatilization due to a lower soil MBC/MBN ratio.

Continuous Cropping and Fertilization on Vertical Distribution of Major Nutrients, SOC Dynamics Through FT-IR Spectroscopy and Developing Soil Quality Indices Under Sandy Clay Loam Soil

ABSTRACT Continuous application of farm yard manure and mineral fertilizer significantly affects the soil available nutrient and distribution of nutrients in soil. Balanced application of mineral fertilization (100% NPK) along with FYM @10 t ha−1 has significantly changed the soil labile C and recalcitrant pool. The study shows that integrated nutrient management has recorded the highest transmittance intensity of hydrophilic bond of carboxyl (1698–1701 cm−1) (3500–3200 cm−1 & 1400 cm−1) and aromatic (1650 cm−1), benzenes and alkenes (2922–2926 cm−1), disulfide (500–400 cm−1), and phosphorus esters (800–580 cm−1) bonds. Soil organic carbon (SOC), soil microbial biomass carbon (SMBC), soil microbial biomass nitrogen (SMBN), labile and recalcitrant pools have observed higher in INM plots than super optimal (150% NPK), optimal (100% NPK), and with micronutrient (100% NPK +Zn). The addition of manure and mineral fertilizers has increased the SOC stocks and SOC sequestration. Soil quality indices were developed using principal component analysis and INM (T8) showed the highest soil quality index which has 10.2 (SQIA) and 10.6 (SQIW) percent higher response than super optimal dose (T3) and other optimal doses (T2, T4, T5 & T9) are on par which each other and lowest in control of intensive farming over four decades. The significant change in sustainable yield index (SYI) of finger millet and maize has observed in plot which received T8 followed by T3, and other optimal doses were least significant with each other and lowest in control plot.

Fertilizer quality and labile soil organic matter fractions are vital for organic carbon sequestration in temperate arable soils within a long-term trial in Switzerland

Agricultural management of soils has led to severe losses of soil organic matter (SOM), accompanied by an increased release of CO2 into the atmosphere and a reduction of soil fertility. Especially under the aspect of global warming and the increasing demand for food, there is a need for sustainable management options increasing soil organic carbon (SOC) storage in agricultural soils, but knowledge gaps exist regarding C persistence in, and its transfer between functional SOC pools, within different farming systems. Here we report on impacts of different farming systems on the temporal dynamics of SOM fractions within the DOK long-term trial (Switzerland), from 1982 to 2017. A purely minerally (CONMIN), a minerally and organically (CONFYM), and a purely organically fertilized farming system (BIODYN) were compared with an unfertilized control (NOFERT). We separated archived soils from the Haplic Luvisol (0–20 cm depth) into particulate (POM) and mineral-associated OM (MAOM) fractions, via physical fractionation, and analyzed the chemical composition of selected fractions via solid-state 13C CPMAS-NMR spectroscopy. We demonstrate that under none of the analyzed farming systems, additional SOC was sequestered in the clay-sized MAOM fraction (<6.3 µm) over a period of 36 years. In all fertilized systems, the amount of SOC in this pool did not change, but strongly decreased in NOFERT (-27%). Bulk SOC increased in BIODYN (+13%) and CONFYM (+5%), but decreased in CONMIN (-8%) and NOFERT (-20%). As no additional SOC accumulated in the clay-sized MAOM fraction, this implies that bulk SOC increases were solely stored within labile POM fractions. NMR spectra showed comparable POM chemical compositions between different systems. Differences in fertilizer quality (BIODYN = composted farmyard manure vs CONFYM = stacked farmyard manure + mineral fertilizer) and the omission of pesticides resulted in better conditions for POM stabilization and consequently significantly higher C contents of occluded POM (oPOM) within aggregates, in BIODYN. However, this labile fraction is at high risk of being lost within a few days, as illustrated by the strong annual oPOM-C content fluctuations depending on the timing of soil sampling after harvest. The highest post-harvest oPOM-C losses in BIODYN indicate the higher dynamics compared to CONFYM. It is anticipated that only sustainable fertilization methods with continuous application of solely organic fertilizers in the long-run can maintain SOC in the labile POM fractions at elevated levels, thereby ensuring soil fertility. It also illustrates the need for prevention of major losses by careful management of the labile POM fractions, as this OM could associate with fine mineral particles at a later stage and thus contribute to OC sequestration in the stable SOC pool. Overall, the potential of arable soils to accumulate stable OC for long-term sequestration is questioned.

Agroforestry perennials reduce nitrous oxide emissions and their live and dead trees increase ecosystem carbon storage.

Agroforestry systems (AFS) contribute to carbon (C) sequestration and reduction in greenhouse gas emissions from agricultural lands. However, previously understudied differences among AFS may underestimate their climate change mitigation potential. In this 3-year field study, we assessed various C stocks and greenhouse gas emissions across two common AFS (hedgerows and shelterbelts) and their component land uses: perennial vegetated areas with and without trees (woodland and grassland, respectively), newly planted saplings in grassland, and adjacent annual cropland in central Alberta, Canada. Between 2018 and 2020 (~April-October), nitrous oxide emissions were 89% lower under perennial vegetation relative to the cropland (0.02 and 0.18gN m-2 year-1 , respectively). In 2020, heterotrophic respiration in the woodland was 53% lower in shelterbelts relative to hedgerows (279 and 600 g C m-2 year-1 , respectively). Within the woodland, deadwood C stock was particularly important in hedgerows (35 MgC ha-1 or 7% of ecosystem C) relative to shelterbelts (2Mg C ha-1 or <1% of ecosystem C), and likely affected C cycling differences between the woodland types by enhancing soil labile C and microbial biomass in hedgerows. Deadwood C stock was positively correlated with annual heterotrophic respiration and total (to ~100 cm depth) soil organic C, water-soluble organic C, and microbial biomass C. Total ecosystem C was 1.90-2.55 times greater within the woodland than all other land uses, with 176, 234, 237, and 449 Mg C ha-1 found in the cropland, grassland, planted saplings treatment, and woodland, respectively. Shelterbelt and hedgerow woodlands contained 2.09 and 3.03 times more C, respectively, than adjacent cropland. Our findings emphasize the importance of AFS for fostering C sequestration and reducing greenhouse gas emissions and, in particular, retaining hedgerows (legacy woodland) and their associated deadwood across temperate agroecosystems will help mitigate climate change.

Prescribed defoliation strategies influence soil carbon and nitrous oxide potential in pastures

AbstractCool‐season perennial grasslands are commonly used for livestock pasture. However, understanding of the belowground effects of forage defoliation (selective pressure applied by grazing) management is nascent. We investigated how defoliation management affects aboveground (forage productivity) and belowground processes (soil C/N), microbial nutrient cycling, and potential nitrous oxide [N2O] emissions). We hypothesized that rotational (moderate frequency) defoliation at low severity will increase soil organic matter (SOM) storage and microbial capacity to cycle C, N, and P, but that these changes would also lead to greater soil N2O emission potential. Data were collected from organically (OF) and conventionally managed (CF) farms with differing botanical composition in West Virginia, where three frequencies and two severities of forage defoliation were imposed. At OF, more severe defoliation decreased labile soil C content (–26%), while moderate defoliation resulted in a 39% reduction in microbial enzymatic capacity to cycle C, but not at CF. The N2O emission potential (ratio of nirK/nosZ functional gene abundance), was differentially affected by defoliation treatments at both sites. At OF, defoliation severity altered N2O emission potential, whereby low severity resulted in an 82% decrease in potential N2O emissions compared with high severity treatments. At CF, continuously defoliated plots exhibited a 71% lower N2O emission potential relative to less frequent defoliation. Belowground microbial parameters were more responsive to defoliation at OF, suggesting past management may influence how grazing management asserts belowground changes. Results suggest that defoliation management could enhance soil C storage, but potentially at the risk of increased N2O emissions.

Limited soil carbon storage under long‐term organic fertilization in solar greenhouses

AbstractEnvironmental conditions, fertilization systems and cultivation practices in greenhouses differ from those in arable soil. Despite being a widespread intensive management practice, large uncertainties exist on the effects of long‐term fertilization on soil organic carbon (SOC) in greenhouse cultivation. The objective of this study was to explore the effects of 11‐year‐long fertilization on carbon (C) stocks in soil aggregates (&gt;2 mm; 2–0.25 mm; 0.25–0.053 mm; &lt;0.053 mm) and SOC fractions (free light fraction (free LF), intra‐aggregate particulate organic carbon (iPOC), mineral‐associated organic carbon (mAOC)) at 0–20 cm and 20–40 cm depth in a solar greenhouse. Five treatments were examined: control (no fertilizer), chicken manure (M), chicken manure plus straw (M + S), chicken manure plus lime (M + Ca), chicken manure plus straw and lime (M + S + Ca). Over the 11 years of continuous intensive organic fertilization, SOC contents of all fertilization treatments first increased sharply and then stabilized after 5 years. The M + S + Ca treatment increased the proportion of aggregate masses, SOC concentrations as well as SOC stocks of soil aggregates &gt;2 mm, 2–0.25 mm, but straw or lime application alone had no significant effects. The M + S + Ca treatment increased the SOC stock of free LF, fine iPOC, coarse iPOC and mAOC by 97%, 38%, 22% and 25% compared with the M treatment, respectively. Straw treatment alone only significantly increased the soil labile C stock (free LF, iPOC). At 0–40 cm depth, the regression between the C stock and the C input had the steepest slope in bulk soil (slope = 0.223, R2 = 0.87**). After 11 years, about 15% of the C inputs were sequestered in the soils. As the mAOC remained largely unaffected, the increase in SOC seems only short‐lived and reaches a new equilibrium within a few years when C inputs are balanced out by mineralization. Consequently, the intensive fertilization and management practices in greenhouse cultivation allowing obtaining high tomato yields results in an overall negative C balance.

High content and distinct spectroscopic characteristics of water-extractable organic matter in rhizosheath soils in a semiarid grassland

Rhizosheaths are sheath-like soils widely observed on plant roots and often have higher bioavailability compared with non-rhizosheath rhizosphere soils. As the most labile soil fraction, the quantity and quality of water-extractable organic matter (WEOM) and its temporal dynamics in rhizosheath soils, however, remain unclear. Here, we used ultraviolet–visible and excitation-emission matrix fluorescence spectroscopy to elucidate the WEOM characteristics of rhizosheath and non-rhizosheath soils in an Inner Mongolian grassland and investigate the temporal dynamics of rhizosheath WEOM from early (June) to late (October) growth stages. The results showed that rhizosheath soils had 88% more bulk WEOM (as indicated by water-extractable organic carbon), 44% more chromophoric WEOM, 55% more humus-like (i.e., humic- and fulvic-like) fluorescent WEOM, and 152% more protein-like fluorescent WEOM compared with non-rhizosheath soils. The absorbance and fluorescence analyses revealed that rhizosheath WEOM exhibited lower aromaticity and molecular size and higher fresh microbial contributions. Further, rhizosheath WEOM was more depleted and showed much higher aromaticity and microbial contributions in the middle (August) growth stage that had the highest temperature and precipitation levels. These findings suggest an enrichment of labile organic compounds, which can serve as the primary contributor to the commonly reported high microbial biomass and activity in rhizosheath soils. Overall, the results supported the exceptionally high labile substrate availability for accelerated microbial carbon processing in the rhizosphere microenvironment of rhizosheath-forming plants.

Cover crops enhance resource availability for soil microorganisms in a pecan orchard

Currently, there are substantial knowledge gaps on the impacts of cover crops on soil resources in tree cropping systems, wherein they are typically planted in interrow alleys and maintained for multiple years. While cover crops uptake soil nutrients and water, they can also prevent soil water evaporative losses and return nutrients to soils via decomposition of plant residues and stimulation of microbial nutrient cycling. This field sampling study examined variances in soil conditions across 5- and 7-year-old, cover cropped pecan orchards. We collected soil beneath cover crops and beneath adjacent trees, where soil was kept bare, to evaluate impacts of cover on the soil biota, nutrients, carbon, and their dynamics across a production season. We employed routine soil chemical analyses, phospholipid fatty acid (PLFA) analysis, and high throughput sequencing of 16 S rRNA genes and ITS regions for soils collected at four time points. We revealed that the cover cropped alley soils contained higher relative abundances of microbes that use labile soil substrates in resource rich conditions than did the tree row soils. Soil chemical analyses provided additional evidence that the cover crops did not deplete soil nutrients and reduce soil moisture, but rather, enhanced soil nutrient and moisture contents during many of the sampling time points. Notably, cover crop plant species correlated with soil nutrients and plant beneficial microbes, which may warrant consideration when selecting cover crop species. The tree row and cover cropped alley soils had different proportions of plant-beneficial mycorrhizal fungi. The tree rows supported higher numbers of ectomycorrhizal (ECM) fungi and alleys had higher relative abundances of arbuscular mycorrhizal (AM) fungi, suggesting potential benefits for tree species like pecan, which support dual colonization by AM and ECM Fungi. Altogether, the cover crops enhanced soil carbon, nutrients, and microbial populations in a pecan orchard and these impacts were frequently larger in a 7-year-old versus 5-year-old orchard.

Interaction of soil microbial communities and phosphorus fractions under long-term fertilization in paddy soil

Understanding the impact of biological activities on the soil phosphorus (P) distribution under long-term fertilizer application can facilitate better soil P fertility management. Therefore, the primary objectives of this study were to investigate the effect of long-term (since 1981) fertilizer application on the soil P fractions and microbial community and to evaluate correlations between the microbial community structure and P distribution. The following treatments were implemented in a long-term field trial: no fertilization (CK), inorganic N and K (NK), inorganic P and K (PK), inorganic N, P and K (NPK) and manure+NPK (MNPK) fertilization. The study showed that the soil pH, soil organic carbon and total and available N and P concentrations were considerably higher in the MNPK treatment than in the CK treatment. The soil microbial biomass C, N and P concentrations were also significantly higher in the MNPK treatment than in the CK treatment. Among fertilization treatments, the β-1,4-glucosidase, α-1,4-glucosidase, urease, acid phosphatase and phosphodiesterase activities were the highest in the MNPK treatment. Compared to inorganic fertilization, the MNPK treatment increased the labile soil P fractions and decreased the residual soil P concentration. Continuous fertilization significantly affected the soil microbial composition. The total phospholipid fatty acid (PLFA) concentrations in the NK, PK, NPK and MNPK treatments were 23.3, 43.1, 48.7 and 87.7% higher, respectively, than in the CK treatment. A significant correlation was observed between the microbial community and soil P fractions. Moreover, the aggregated boosted tree (ABT) model showed that among the various soil biochemical properties, the total PLFA concentration was the factor that most influenced the active P pool, accounting for 35.4% of the relative influence of all soil biochemical properties examined. These findings reveal that combined manure and inorganic fertilizer application is a better approach than applying inorganic fertilizer alone for sustaining long-term P fertility by mediating soil biological activity.

Effects of global change and human disturbance on soil carbon cycling in boreal forest: A review

Increasing human demands for Earth's resources are hastening many environmental changes and creating a need to incorporate the routine monitoring of ecosystem functions into forest management. Under global change and anthropogenic disturbances, soil carbon (C) cycling in terrestrial ecosystems is undergoing substantial changes that result in the transformation between soil C sources and sinks. Therefore, the forest C budget requires an understanding of the underlying soil C dynamic under environmental disturbances. The present review focuses on the response and feedback of soil C cycling to global change (climate warming and nitrogen (N) deposition) and human disturbances (fire and logging) and detects the association of soil C cycling with soil C and N efflux and inflow in boreal forests. The effects of climate warming and N deposition on soil C cycling are complex, especially at short-term temporal scales. Climate warming can decay soil organic matter (SOM) to emit substantial amounts of CO2, and differing warming durations result in different effects on soil C loss, ranging from ca. 1 to 15 Mg C ha–1. Short-term soil warming mainly reduces the labile soil C pool and increases the decomposition of recalcitrant soil C compounds (e.g., lignin), whereas longer-term warming may limit soil C loss due to impoverished soil C substrate and microbial communities. Moderate N addition is conducive to enhancing soil C storage (ca. 2–22 Mg C ha–1), by increasing plant productivity including above- and belowground biomass; however, chronic N deposition or excess N addition can result in soil acidity, reducing N use efficiency and plant growth and further resulting in no changes or declines in soil C pool. Fire and logging lead to a large quantity of soil C loss via impaired plant productivity and increased organic matter degradation, exacerbating global warming. In particular, severe fire can cause a large amount of soil C loss, ca. 16–34 Mg C ha–1 in the data we reviewed. Meanwhile, the black C input induced by fire and the plant residual C input from the roots of logged trees can increase the proportion of recalcitrant soil C and enhance the stability of soil C pool. We also highlight the positive feedback of forest restoration to soil C storage after fire and logging disturbances, indicating that effective forest restoration projects (e.g., afforestation and natural forest recovery) are necessary to sequester soil C belowground. Additionally, combined with microbial technologies and metagenomics- and metabolomics-based approaches, soil microorganisms are proved crucial for driving soil C cycling via C capture and the N recycling of plants and soil. We, therefore, suggest that clarifying the relationship among plant, SOM, and microorganisms is essential to better evaluate soil C cycling and to predict how boreal forests respond to global change and human disturbances. Further work is needed to assess long-term soil C feedback from high-latitude forests to broader regions.