Belowground Inputs Research Articles

Roots are the key for soil C restoration: A comparison of land management in the semiarid Argentinean Pampa

The objective of the study was to elucidate the relationship between soil management and carbon (C) stocks, and to identify the factors that intervene in the processes that favor C sequestration. The study was carried out on a farm in the Argentinean semiarid pampas with four land management practices (2019–2021): natural grassland (NG), crop-pasture rotation (RO), soybean monoculture (S-S) and with cover crop (S-CC). Aerial and root biomass were quantified at cover crop termination and soybean flowering. In addition, residues on soil surface were determined two times a year. In all cases, C and N contents were quantified. On composite soil samples, soil organic C (SOC), particulate C (POC), microbial biomass C and N (MBC, MBN), and soluble C and N were determined at 0–0.1 m depth. Results showed that NG had the highest and the most stable aboveground (3807.1 kg residue-C ha-1) and belowground inputs between years (4965.8 kg root-C ha-1). Similar results were observed in RO treatment during the first year (4221.0 kg root-C ha-1) diminishing by 67% after plowing for the annual crop in the second year reaching similar root-C values than S-CC and S-S (721.7 kg ha-1). S-S presented the lowest aboveground (49%) and belowground inputs (77%) compared to NG. Cover crops (S-CC) contributed with 31% and 14% of extra residue-C and root-C, respectively, compared to S-S. MBC showed a non-linear response with increases in root-C, reaching maximum carrying capacity of 110.6 ± 4.3 kg MBC ha-1 with root-C inputs ≥ 2200 kg ha-1. Our results showed significant relationships between root-C and POC and SOC, while no relationships were found for aboveground residues. Increases in soluble N explained 86% of SOC variability. Both RO and S-CC reached the “4 per Mille” goal with an average annual SOC storage rate (ΔC) of 0.24 and 0.16 Mg ha-1 y-1, respectively, while S-S had SOC losses of 0.04 Mg ha-1yr-1.

Cover crop root morphology rather than quality controls the fate of root and rhizodeposition C into distinct soil C pools.

Cover crops increase carbon (C) inputs to agricultural soils, and thus have the potential to mitigate climate change through enhanced soil organic carbon (SOC) storage. However, few studies have explored the fate of belowground C inputs associated with varying root traits into the distinct SOC pools of mineral-associated organic carbon (MAOC) particulate organic carbon (POC). Therefore, a packed 0.5 m column trial was established with 0.25 m topsoil and 0.25 m subsoil with four cover crops species (winter rye, oilseed radish, chicory, and hairy vetch) known to differ in C:N ratio and root morphology. Cover crops were 14 CO2 -labeled for 3 months, and then, half of the columns were sampled to quantify root and rhizodeposition C. In the remaining columns, plant shoots were harvested and the undisturbed soil and roots were left for incubation. Bulk soil from both sampling times was subjected to a simple fractionation scheme, where 14 C in the <50 and >50 μm fraction was assumed to represent MAOC and POC, respectively. The fast-growing rye and radish produced the highest root C. The percentage loss of C via rhizodeposition (%ClvR) showed a distinct pattern, with 22% for the more branched roots (rye and vetch) and 6%-8% for the less branched roots (radish and chicory). This suggests that root morphology plays a key role in determining rhizodeposition C. After 1 year of incubation at room temperature, the remaining MAOC and POC were positively correlated with belowground inputs in absolute terms. However, topsoil MAOC formation efficiencies (cover crop-derived MAOC remaining as a share of belowground inputs) were higher for vetch and rye (21% and 15%, respectively) than for chicory and radish (9% and 10%, respectively), suggesting a greater importance of rhizodeposition (or indirectly, root morphology) than solely substrate C:N ratio for longer term C stabilization.

Individual and interactive effects of warming and nitrogen supply on CO2 fluxes and carbon allocation in subarctic grassland.

Climate warming has been suggested to impact high latitude grasslands severely, potentially causing considerable carbon (C) losses from soil. Warming can also stimulate nitrogen (N) turnover, but it is largely unclear whether and how altered N availability impacts belowground C dynamics. Even less is known about the individual and interactive effects of warming and N availability on the fate of recently photosynthesized C in soil. On a 10-year geothermal warming gradient in Iceland, we studied the effects of soil warming and N addition on CO2 fluxes and the fate of recently photosynthesized C through CO2 flux measurements and a 13 CO2 pulse-labeling experiment. Under warming, ecosystem respiration exceeded maximum gross primary productivity, causing increased net CO2 emissions. N addition treatments revealed that, surprisingly, the plants in the warmed soil were N limited, which constrained primary productivity and decreased recently assimilated C in shoots and roots. In soil, microbes were increasingly C limited under warming and increased microbial uptake of recent C. Soil respiration was increased by warming and was fueled by increased belowground inputs and turnover of recently photosynthesized C. Our findings suggest that a decade of warming seemed to have induced a N limitation in plants and a C limitation by soil microbes. This caused a decrease in net ecosystem CO2 uptake and accelerated the respiratory release of photosynthesized C, which decreased the C sequestration potential of the grassland. Our study highlights the importance of belowground C allocation and C-N interactions in the C dynamics of subarctic ecosystems in a warmer world.

Energy cover crops for biogas production increase soil organic carbon stocks: A modeling approach

AbstractEnergy cover crops for biogas production through anaerobic digestion (AD) are inserted between two primary crops. They replace either bare soil or nonharvested cover crops, and their management is usually intensified to produce more biomass. They allow the production of renewable energy as well as digestate, used as an organic fertilizer, without directly competing with food production. Because of the increased biomass production and export and of the return of a digested biomass to the soil, the impact of energy cover crops on soil organic carbon (SOC) is questioned. The objective of this paper was to study the difference in SOC stocks induced by the introduction of energy cover crops for AD coupled with the application of the resulting amount of digestate. We used the AD model Sys‐Metha combined with the soil C model AMG to simulate SOC stocks for 13 case studies in France, with scenarios comparing different intercrop management practices, with or without cover crops, harvested or not. Our results indicated that the higher biomass production of energy cover crops (from 6.7 to 11.1 t DM ha−1) in comparison with nonharvested cover crops (2 t DM ha−1) or bare soil led to higher humified C input (belowground input and digestate), despite the high C fraction exported in AD. This resulted in an increase in SOC stocks in comparison with nonharvested cover crops or bare soil (from 0.01 to 0.12 t C ha−1 year−1 over 30 years). The uncertainties in the model parameters did not modify these results. However, in the case of equal biomass production between energy cover crops and nonharvested cover crops, SOC stocks would be lower with energy cover crops.

Climate, Soil, and Plant Controls on Early-Stage Litter Decomposition in Moso Bamboo Stands at a Regional Scale

Moso bamboo (Phyllostachys edulis) is currently distributed across a wide geographical area in East Asia. As a common bamboo species occurring along a broad environmental gradient, there is a need to understand how environmental and biotic drivers affect belowground processes at large scales. In this study, we investigated the influence of climate, soil properties, stand characteristics, and organic matter input parameters as potential drivers of the initial decomposition process in Moso bamboo stands at a regional scale. Using the Tea Bag Index method, we estimated the initial decomposition rate (k) and stabilization factor (S; potential long-term carbon storage) from standard litter incubated at 13 sites across southern Japan and Taiwan. We found that both decomposition parameters were strongly affected by the climate. The climatic conditions during the incubation period better explained the variance in k. In contrast, the long-term climate was more important for S. Notably, temperature and precipitation interactively affected the initial decomposition rates. This interaction showed that in warmer sites, precipitation increased k, whereas in cooler sites, precipitation had no effect or even decreased k. Soil parameters had no influence on k and only had minor effects on S. A structural equation model showed that the stabilization factor was indirectly affected by stand density, which suggests that higher bamboo densities could increase litter stabilization by increasing above-and below-ground organic matter input. Our study highlights the central role of climate in controlling decomposition processes in Moso bamboo stands on a broad scale. Moreover, differences in stand structure can indirectly affect potential soil carbon storage through changes in organic matter input and soil conditions.

Belowground plant inputs exert higher metabolic activities and carbon use efficiency of soil nematodes than aboveground inputs

Soil nematodes are key components of soil food web and, through their metabolic activities, play a crucial role in soil carbon (C) cycling. Aboveground and belowground plant C inputs can directly, or indirectly via soil microbes, modify nematode abundance and community composition. Aboveground and belowground C inputs differ in chemical composition, amounts, and frequency, so we hypothesized that the two input pathways affect nematode communities differently. To assess the relative contributions of aboveground versus belowground inputs to nematode community composition and activity, we subjected grassland soils to four plant input pathways over two consecutive years: no input, only aboveground input (+A), only belowground input (+B), and both aboveground and belowground inputs (+A + B). Nematode metabolic footprints, as estimates of C used in growth/reproduction and C lost by respiration, and C use efficiency (C used/(C used + C lost)) were calculated. We predicted that soils with belowground inputs, which are more directly linked to the soil biota, and which contain a more labile blend of molecules, would support richer and more complex nematode communities, and also favor a bacterial-driven decomposition channel. Accordingly, we showed that + B soils supported higher nematode numbers than + A soils, and that the bacterial decomposition channel was dominant in + B soils, while the fungal decomposition channel dominated in + A soils. Compared with + A soils, +B input system increased nematode structure footprints (the metabolic footprints of nematodes in upper functional guilds) rather than enrichment footprints (the metabolic footprints of enrichment opportunistic nematodes). Moreover, we observed that, compared to + A soils, +B soils had higher growth and respiration rates of bacterivores, omnivores-predators, and total nematodes. Finally, we found higher C use efficiency values for omnivores-predators and total nematodes in + B than in + A soils. We thus conclude that belowground plant-derived resources, by changing the ratio between fungivores and bacterivores, induce a faster carbon turnover rate, and higher metabolic activity of soil nematodes within soil food web, ultimately spurring richer and more efficient soil food web than aboveground inputs.

Durum wheat salt stress tolerance is modulated by the interaction between plant genotypes, soil microbial biomass, and enzyme activity

Understanding the relationship between durum wheat genotypes and soil biochemistry under salt stress plays a key role in breeding for yield superior genotypes. Thus, microbial biomass carbon (MBC) and nitrogen (MBN), the activity of three selected enzymes including dehydrogenase (D-ase), alkaline phosphatase (Alk-ase), and protease (P-ase), and available phosphorus (available P) and nitrogen (available N) were assessed. Two landraces and two improved varieties were tested under two salinity levels of water irrigation (0.3 and 12 dS m–1). Soil sampling was carried out at five durum wheat growth stages. The soil biota-genotype interaction seems to affect the biological (MBC, MBN, and enzymatic activities) and chemical (available P and N) traits. The microbial activity of rhizospheric soil was higher at the tillering and flowering stages. Under saline conditions, ‘Maali’ (improved variety) and ‘Agili Glabre’ (landrace) showed the best belowground inputs (e.g., MBC, MBN, enzymatic activities, available P and N) and grain yield (GY) performance. Under the same conditions, four soil biochemical indicators of GY of tolerant genotypes (i.e., ‘Maali’ and ‘Agili Glabre’) were determined as available N, P-ase, available P, Alkase, and D-ase. Stepwise analysis revealed that predictive variables depended on growth stages. Overall, MBC, available N, Alk-ase, and P-ase were the variables that mainly contributed to predicting GY in saline environments. In conclusion, the results suggested a specific interaction between plant genotype roots and soil microbes to overcome salt stress. Thus, soil biological components should acquire more importance in plant salinity tolerance studies. Highlights- Salt-tolerant durum wheat genotypes showed greater microbial activities in the rhizosphere.- Microbial enzymatic changes depended on the interaction plant genotype x soil salinity.- The MBC/MBN ratio and dehydrogenase strongly correlated with grain yield under salinity.- MBC, available N, and alkaline phosphatase as predictors of grain yield at 12 dS m–1.- Tillering and flowering could be key stages of durum wheat salinity tolerance.

Aboveground litter addition for five years changes the chemical composition of soil organic matter in a temperate deciduous forest

Global climate change has been found to affect plant net primary production and thereby could modify the quantity and quality of aboveground litter, which may consequently affect soil organic carbon (SOC). Whether those changes have influenced the chemical components of SOC is still unclear, especially for the soil beneath the surface soil. This study examined the changes in biopolymers (lignin, substituted fatty acids, amino sugars) in 0–10 cm and 10–20 cm soil depth in a temperate forest after five years of aboveground litter addition or removal treatments. The results showed that although SOC and soil nitrogen contents were not influenced by litter manipulation, the lignin phenols and substituted fatty acids contents in SOC of 10–20 cm soil were significantly higher in litter addition treatment compared to litter removal treatment, suggesting a selective accumulation pathway for these plant biopolymers in the 10–20 cm soil under litter manipulation. Soil amino sugar content, however, was not influenced by litter manipulation, although microbial biomass carbon (MBC) and nitrogen in 0–10 cm soil were significantly higher in litter addition plots due to the significantly higher substrate availability (dissolved organic carbon) and nutrient availability (total dissolved nitrogen) than those in the control. Although higher MBC could increase decomposition of SOC, this effect did not counterbalance the effect of higher aboveground litter input in litter addition plots, but rather changed the composition of SOC toward compounds of a purportedly more stable composition. The effects of aboveground litter removal were not entirely in contrast to the effects of aboveground litter addition on SOC, hinting that belowground inputs could sustain the storage of SOC for at least five years under aboveground litter removal treatment. The changes in the chemical composition of SOC under litter manipulation for five years suggested the potential changes in SOC stability and pool size in the future, especially in the 10–20 cm soil, to which more attention should be paid in future litter manipulation studies.

Importance of resilient pastures for New Zealand’s agricultural soil carbon stocks

New Zealand’s agricultural pastures contain significant soil carbon (C) stocks that are susceptible to change when impacted by management and natural processes (e.g., climate). Inputs of C to these pastoral soils is through photosynthetic uptake of atmospheric CO2 either on-site or elsewhere. Changes in soil C stocks are in response to the management of the system that alters the input-output balance. Increasing the resilience of pastures to climatic events such as hot and dry summers or cool and wet winters can increase inputs of C to the soil while sustaining above-ground production and so provide an opportunity for C sequestration. Furthermore, increased pasture for grazing can reduce the need for management practices identified as detrimental for soil C stocks such as irrigation or the production of cropped supplemental feed. A reduction in the need for renewal and its associated soil C losses, and the establishment of a more diverse sward, especially if deeper-rooting species are included, has the potential for increasing soil C stocks provided the diversity can be maintained. From a soil C perspective, a resilient pasture maximises CO2 uptake to ensure adequate above- and below-ground inputs to maintain or increase soil C stocks and minimise the need for management activities detrimental to soil C.

No changes in soil organic carbon and nitrogen following long-term prescribed burning and livestock exclusion in the Sudan-savanna woodlands of Burkina Faso

Fire and overgrazing reduce aboveground biomass, leading to land degradation and potential impacts on soil organic carbon (SOC) and total nitrogen (TN) dynamics. However, empirical data are lacking on how prescribed burning and livestock exclusion impact SOC in the long-term. Here we analyse the effects of 19 years of prescribed annual burning and livestock exclusion on tree density, SOC and TN concentrations in the Sudanian savanna ecoregion at two sites (Tiogo and Laba) in Burkina Faso. Results revealed that neither livestock exclusion nor prescribed burning had significant impact on SOC and TN concentrations. The results at both sites indicate that 19 years of livestock and fire exclusion did not result in a significant increase in tree density compared to grazing and annual prescribed burning. The overall mean (± SEM) of SOC stocks in the 0–50 cm depth increment in the unburnt (53.5 ± 4.7 Mg C ha−1) and annually burnt (56.4 ± 4.3 Mg C ha−1) plots at Tiogo were not statistically different. Similarly, at Laba there was no significant difference between the corresponding figures in the unburnt (37.9 ± 2.6 Mg ha−1) and in the annually burnt plots (38.6 ± 1.9 Mg ha−1). Increases in belowground inputs from root turnover may have countered changes in aboveground biomass, resulting in no net change in SOC and TN. We conclude that, contrary to our expectation and current policy recommendations, restricting burning or grazing did not result in increase in SOC stocks in this dry savanna ecosystem.

Simulating measurable ecosystem carbon and nitrogen dynamics with the mechanistically defined MEMS 2.0 model

Abstract. For decades, predominant soil biogeochemical models have used conceptual soil organic matter (SOM) pools and only simulated them to a shallow depth in soil. Efforts to overcome these limitations have prompted the development of the new generation SOM models, including MEMS 1.0, which represents measurable biophysical SOM fractions, over the entire root zone, and embodies recent understanding of the processes that govern SOM dynamics. Here we present the result of continued development of the MEMS model, version 2.0. MEMS 2.0 is a full ecosystem model with modules simulating plant growth with above- and belowground inputs, soil water and temperature by layer, decomposition of plant inputs and SOM, and mineralization and immobilization of nitrogen (N). The model simulates two commonly measured SOM pools – particulate and mineral-associated organic matter (POM and MAOM, respectively). We present results of calibration and validation of the model with several grassland sites in the US. MEMS 2.0 generally captured the soil carbon (C) stocks (R2 of 0.89 and 0.6 for calibration and validation, respectively) and their distributions between POM and MAOM throughout the entire soil profile. The simulated soil N matches measurements but with lower accuracy (R2 of 0.73 and 0.31 for calibration and validation of total N in SOM, respectively) than for soil C. Simulated soil water and temperature were compared with measurements, and the accuracy is comparable to the other commonly used models. The seasonal variation in gross primary production (GPP; R2 = 0.83), ecosystem respiration (ER; R2 = 0.89), net ecosystem exchange (NEE; R2 = 0.67), and evapotranspiration (ET; R2 = 0.71) was well captured by the model. We will further develop the model to represent forest and agricultural systems and improve it to incorporate new understanding of SOM decomposition.

Effects of aboveground and belowground litter inputs on the balance of soil new and old organic carbon under the typical forests in subtropical region.

Litter is one of the most important factors controlling the accumulation, stabilization, and turnover of soil organic carbon (SOC) in forests. There is a knowledge gap of the impacts of aboveground and belowground litter inputs on the balance of new and old SOC under different forests in subtropical region. We examined the effects of aboveground and belowground litter inputs on SOC turnover using isotopic tracing technique, based on a 3-year C3 plants/C4 soil replacement experiment in natural forest (NF), Masson pine (Pinus massoniana) plantation (PM) and Chinese fir (Cunninghamia lanceolata) plantation (CL). Our results showed that forest types, litter treatments, and sampling time significantly affected SOC contents, δ13C, new and old SOC contents. Moreover, there were significant interactions between forest types and litter treatments. Litter input increased SOC content and net SOC increment, with higher sensitivity of NF than CL. Litter inputs decreased soil δ13C, with lower values in NF and PM compared to CL. For PM, the new SOC content in belowground litter treatment was significantly higher than that in aboveground litter treatment. The contents of old SOC were lower in belowground litter treatment than aboveground litter treatment in the NF and CL. Above- and below-ground biomass were positively correlated with SOC content and net increment. Belowground litter biomass were positively correlated with soil C/N ratio and new SOC content. Our results implied that belowground litter input had stronger effects on SOC turnover compared to aboveground litter input, with the effects varying among different forests. Our results provided new information on SOC accumulation and on sustainable management of the typical forests in subtropical region.

Land-use perturbations in ley grassland decouple the degradation of ancient soil organic matter from the storage of newly derived carbon inputs

Abstract. In a context of global change, soil has been identified as a potential carbon (C) sink, depending on land-use strategies. To detect the trends in carbon stocks after the implementation of new agricultural practices, early indicators, which can highlight changes in short timescales, are required. This study proposes the combined use of stable isotope probing and chemometrics applied to solid-state 13C nuclear magnetic resonance (NMR) spectra to unveil the dynamics of the storage and mineralization of soil carbon (C) pools. We focused on light organic matter fractions isolated by density fractionation of soil water stable aggregates because they respond faster to changes in land use than the total soil organic matter (SOM). Samples were collected from an agricultural field experiment with grassland, continuous maize cropping, and ley grassland under temperate climate conditions. Our results indicated contrasting aggregate dynamics depending on land-use systems. Under our experimental conditions, grassland returns larger amounts of C as belowground inputs than maize cropping, evidencing a different distribution of light C fractions between aggregate classes. Coarse aboveground inputs from maize contributed mostly to larger macroaggregates. Land-use changes with the introduction of ley grassland provoked a decoupling of the storage and/or degradation processes after the grassland phase. The newly derived maize inputs were barely degraded during the first 3 years of maize cropping, whereas grassland-derived material was depleted. As a whole, results suggest large microbial proliferation as shown by 13C NMR under permanent grassland, then reduced within the first years after the land-use conversion, and finally restored. The study highlighted a fractal structure of the soil, determining a scattered spatial distribution of the cycles of storage and degradation of soil organic matter related to detritusphere dynamics. As a consequence, vegetal inputs from a new land use are creating new detritusphere microenvironments that may be disconnected from the dynamics of C cycle of the previous land use. The formation of those different and unconnected microenvironments may explain the observed legacy effect of the previous land use, since each microenvironment type contributes separately to the overall soil C cycle. The effects of the new land use on the soil C cycle are delayed until the different detritusphere microenvironments remain unconnected, and the ones from the previous land use represent the predominant microenvironment type. Increasing knowledge of the soil C dynamics at a fine scale will be helpful in refining the prediction models and land-use policies.

Soil C storage following salvage logging and residue management in bark beetle-infested lodgepole pine forests

Bark beetle outbreaks have altered carbon (C) dynamics in lodgepole pine (Pinus contorta var. latifolia Engelm. ex Wats.) forests across western North America. The sensitivity of soil C to post-beetle management operations remains unknown in these forests. We quantified total C stocks in the O horizon and upper mineral soil (0–10 cm) 6–7 years after salvage logging in lodgepole pine forests of northern Colorado and compared biomass retention or removal residue management to uncut beetle-infested forests. We evaluated the quantity and chemical composition of C in physical fractions of the O horizon and upper mineral soil layers to better understand how management impacts the quality of C with implications for soil C persistence. Post-harvest residue retention increased O horizon C storage compared to the removal treatment and uncut stands. Total soil C (O horizon and upper 0–10 cm mineral soil) was an average of 11.4 (±5.6) Mg C ha−1 higher in salvage logged plots compared to uncut forests and the increase was primarily in particulate organic matter. Salvage logging did not alter the chemical composition of the soil C fractions analyzed. The C:N ratios, Fourier-transform mid-infrared spectra, and 13C values analyses each showed that the C composition of the organic and upper mineral soil layers were distinct. This indicates that physical transfer of fresh or decomposed O horizon material may not be the primary source of particulate C, and we suggest that belowground inputs are likely to be an important contributor to these mineral soils. Our finding that logging residue removal did not reduce post-harvest upper mineral soil C stocks has implications for biomass harvesting in these forests, though the long-term impacts of post-bark beetle management on C storage in these ecosystems depend on understory dynamics, tree regeneration and forest recovery that will progress over the course of decades. This work suggests that greater understanding of post-logging organic matter inputs and C dynamics in O horizon and mineral SOM fractions can help inform forest management aimed at sustaining soil C.

Whole-profile soil organic matter content, composition, and stability under cropping systems that differ in belowground inputs

Subsoils have been identified as a potential carbon sink because they typically have low soil organic carbon (SOC) concentrations and high SOC stability. One proposed strategy to increase SOC stocks is to enhance C inputs to the subsoil by increasing crop rotation diversity with deep-rooted perennial crops. Using three long-term field trials in Iowa (study durations of 60, 35, and 12 years), we examined the effects of contrasting cropping systems [maize (Zea mays L.)-soybean (Glycine max (L.) Merr) (= two-year system) vs. maize-soybean-oat (Avena sativa L.)/alfalfa (Medicago sativa L.)-alfalfa or maize-maize-oat/alfalfa-alfalfa (= four-year system)] on above- and below-ground C inputs, as well as the content, biochemical composition, and distribution of SOC among physical fractions differing in stability to 90 cm depth. Average annual total C inputs were similar in the two-year and four-year systems, but the proportion of C delivered belowground was 20–35 % greater in the four-year system. Despite the long duration of these studies, the effect of cropping system on SOC content to 90 cm was inconsistent across trials, ranging from −7 % to +16 % in the four-year relative to the two-year system. At the one site where SOC was significantly greater in the four-year system, the effect of cropping system on SOC content was observed in surface and subsoil layers rather than limited to the subsoil (i.e., below 30 cm). Cropping system had minimal effects on biochemical indicators of plant-derived organic matter or on the proportions of SOC in labile particulate organic matter versus stable mineral-associated organic matter. We conclude that adoption of cropping systems with enhanced belowground C inputs may increase total profile SOC, but the effect is minimal and inconsistent; furthermore, it has minor impact on the vertical distribution, biochemical composition, and stability of SOC in Mollisols of the Midwest U.S.

Spatially resolved soil solution chemistry in a central European atmospherically polluted high-elevation catchment

Abstract. We collected soil solutions by suction lysimeters in a central European temperate forest with a history of acidification-related spruce die-back in order to interpret spatial patterns of soil nutrient partitioning, compare them with stream water chemistry and evaluate these parameters relative to concurrent loads of anions and cations in precipitation. Five lysimeter nests were installed in the 33 ha U dvou loucek (UDL) mountain catchment at different topographic positions (hilltops, slopes and valley). Following equilibration, monthly soil solution samples were interrogated over a 2-year period with regard to their SO42-, NO3-, NH4+, Na+, K+, Ca2+, Mg2+ and total dissolved Al concentrations, organic carbon (DOC) and pH. Soil pits were excavated in the vicinity of each lysimeter nest to also constrain soil chemistry. For an estimation of phosphorus (P) availability, ammonium oxalate extraction of soil samples was performed. Cation exchange capacity (CEC ≤58 meq kg−1) and base saturation (BS ≤13 %) were found to be significantly lower at UDL than in other monitored central European small catchments areas. Spatial trends and seasonality in soil solution chemistry support belowground inputs from mineral-stabilized legacy pollutants. Overall, the soil solution data suggest that the ecosystem was still chemically out of balance relative to the concurrent loads of anions and cations in precipitation, documenting incomplete recovery from acidification. Nearly 30 years after peak acidification, UDL exhibited similar soil solution concentrations of SO42, Ca2+ and Mg2+ as median values at the Pan-European International Co-operative Program (ICP) Forest sites with similar bedrock lithology and vegetation cover, yet NO3- concentrations were an order of magnitude higher. When concentrations of SO42-, NO3- and base cations in runoff are compared to soil pore waters, higher concentration in runoff points to lateral surficial leaching of pollutants and nutrients in excess than from topsoil to subsoil. With P availability being below the lowest range observed in soil plots from the Czech Republic, the managed forest ecosystem in UDL probably reflects growing inputs of C from regenerating vegetation in the N-saturated soil, which leads to P depletion in the soil. In addition, the observed spatial variability provides evidence pointing to substrate variability, C and P bioavailability, and landscape as major controls over base metal leaching toward the subsoil level in N-saturated catchments.

Stable carbon in soils under rubber tree (Hevea brasiliensis) agroforestry systems in the south of Bahia, Brazil

The early natural forest conversion to pasture, rubber tree plantations or agroforestry systems (AFSs) will not negatively affect soil organic carbon (SOC) accumulation due to the lack of soil tillage and the continuous contribution of vegetal residues to the soil, over the years, which would favor C occlusion in soil aggregates. The objectives of this study were: to evaluate the potential of AFSs with rubber trees to accumulate SOC up to 100 cm; to evaluate the δ13C variations over the soil profile after forest to pasture and rubber tree plantation (RTP) conversion; and to verify the C physical protection in aggregates as a mechanism of C stability in these soils. SOC was quantified up to 100 cm in whole soil and in three fraction-size classes (macroaggregates, microaggregates and silt + clay class). The occluded C was quantified in macro- and microaggregates by using an ultrasonic method. The relative contributions of C3 and C4 plants-derived C were quantified. The RTP system presented the highest SOC values, up to 100 cm depth, contributing with 20 Mg C ha−1 from belowground input, during 35-year old of system implantation. However, about 90% of SOC was not physically protected in the soil aggregates. The forest to rubber + cacao agroforestry system (AFS) conversion reduced SOC stock up to 100 cm approximately 72 Mg ha−1, and the reduction in SOC stock after forest to rubber tree + acai AFS was around 34 Mg ha−1. Rubber tree + cacao and rubber tree + acai AFSs had significant contributions of occluded C in soil aggregates.

Soil enzyme activities increase following restoration of degraded subtropical forests

Ecological restoration increases biodiversity and carbon and nitrogen accumulation. Although soil enzyme activities are critical to element cycling and plant diversity and productivity in natural habitats, little is known regarding the effects of ecological restoration on the activities of soil resident enzymes. We investigated the influence of ecosystem restoration on microbial carbon and nitrogen dynamics in soil, as well as the functionality of five enzymes (catalase, dehydrogenase, polyphenol oxidase, urease, and invertase) via the extraction of samples from stands that were subjected to continuous anthropogenic disturbances (i.e., recurring fuelwood collection and domestic animal grazing) in contrast to samples obtained from stands which were left undisturbed for 6, 11, 21, and 31 years in the subtropical forests of China. We found that soil microbial biomass carbon and nitrogen increased with time since restoration, and the activities of catalase, dehydrogenase, invertase, urease, and polyphenol oxidase were amplified in the undisturbed stands following 11 years of restoration, which continued at high levels or decreased thereafter. Redundancy analysis revealed that enzyme activities were augmented in response to biomass inputs from aboveground litterfall and fine root mortality and were positively related to soil carbon and available nitrogen, phosphorus, and potassium contents. Our results highlight the multiple links between increased aboveground and belowground inputs, the abundance of soil-dwelling microorganisms and their enzyme activities, and their contributions to soil carbon and nutrient cycling following the ecological restoration of degraded subtropical forests.

Differences in N2O Fluxes and Denitrification Gene Abundance in the Wet and Dry Seasons Through Soil and Plant Residue Characteristics of Tropical Tree Crops

The conversion of forest to agricultural soils is a widespread activity in tropical systems, and its link to nitrous oxide (N2O) fluxes and nitrogen cycling gene abundance is relevant to understand environmental drivers that may interact with climate change. A current challenge to estimating N2O emissions from land use conversion is an incomplete understanding of crop-specific impacts on denitrifier communities and the N2O fluxes driven by differences in the above- and below-ground inputs with crop type. To address this knowledge gap in tree crops, we evaluated N2O fluxes and denitrification gene abundance and their relationships with soil and plant residue characteristics in citrus and eucalyptus plantations in the field and in soil incubations. We found that the accumulated N2O fluxes from soil were lower for the two agricultural field sites than those for their adjacent forest sites in dry and wet seasons. The N2O fluxes were higher in the wet season, and this seasonal difference persisted even when the soils collected from both seasons were incubated under the same moisture and temperature conditions in the lab for 30 days. Increased N2O fluxes in the wet season were accompanied by an increase in soil nirK and nosZ gene abundance, the dissolved organic carbon (DOC) concentration, and the total soil carbon (C) and nitrogen (N) content. In turn, the abundance of denitrifiers, as indicated by nirK, nirS and nosZ gene copy numbers, showed a low but significant positive correlation with soil bulk density. Our results suggest that soil moisture, leaf litter and crop residues influence the seasonal differences in both N2O fluxes and abundance of denitrifiers in citrus- and eucalyptus-cultivated soils, likely through effects on soil physicochemical characteristics. These findings highlight the overwhelming role of environmental drivers that can make investigating microbial drivers difficult in the field and open the possibility for a better understanding of N cycling processes in tropical soils based on paired field- and incubation-based experimentation.

Microbial formation of stable soil carbon is more efficient from belowground than aboveground input

The relative contributions of aboveground versus belowground plant carbon inputs to the stable soil organic carbon pool are the subject of much debate—with direct implications for how the carbon cycle is modelled and managed. The belowground rhizosphere pathway (that is, carbon exiting the living root) is theorized to form stable soil carbon more efficiently than the aboveground pathway. However, while several mechanisms have been invoked to explain this efficiency, few have been empirically tested or quantified. Here, we use soil microcosms with standardized carbon inputs to investigate three posited mechanisms that differentiate aboveground from belowground input pathways of dissolved organic carbon—through the microbial biomass—to the mineral-stabilized soil organic carbon pool: (1) the physical distance travelled, (2) the microbial abundance in the region in which a carbon compound enters (that is, rhizosphere versus bulk soil) and (3) the frequency and volume of carbon delivery (that is, infrequent ‘pulse’ versus frequent ‘drip’). We demonstrate that through the microbial formation pathway, belowground inputs form mineral-stabilized soil carbon more efficiently than aboveground inputs, partly due to the greater efficiency of formation by the rhizosphere microbial community relative to the bulk soil community. However, we show that because the bulk soil has greater capacity to form mineral-stabilized soil carbon due to its greater overall volume, the relative contributions of aboveground versus belowground carbon inputs depend strongly on the ratio of rhizosphere to bulk soil. Belowground carbon inputs form stable soil carbon more efficiently through microbial formation than carbon addition aboveground, according to soil microcosm experiments that quantitatively compare soil carbon formation efficiencies from different mechanistic pathways.