Loss Of Soil Organic Matter Research Articles

What Agriculture Can Learn from Native Ecosystems in Building Soil Organic Matter: A Review

Over the last century, researchers and practitioners with diverse backgrounds have articulated the importance of improving soil organic matter (SOM) contents in agricultural soils. More recently, climate change scientists interested in CO2 sinks, and agroecologists interested in ecological intensification have converged on the goal of building SOM stocks in croplands. The challenge is that agriculture itself is responsible for dramatic losses of SOM. When grassland or forest ecosystems are first converted to agriculture, multiple mechanisms result in SOM declines of between 20% and 70%. Two of the most important mechanisms are the reduction in organic matter inputs from roots following the replacement of perennial vegetation with annual crop species, and increases in microbial respiration when tillage breaks open soil aggregates exposing previously protected organic matter. Many agricultural practices such as conservation tillage and integration of cover crops have been shown to achieve some degree of SOM improvement, but in general adoption of these practices falls short of accumulating the SOM stocks maintained by grasslands, forests or other native ecosystems that agriculture replaced. Two of the overarching reasons why native terrestrial ecosystems have achieved greater soil organic matter levels than human agroecosystems are because they direct a greater percentage of productivity belowground in perennial roots, and they do not require frequent soil disturbance. A growing body of research including that presented in this review suggests that developing perennial grain agroecosystems may hold the greatest promise for agriculture to approach the SOM levels that accumulate in native ecosystems. We present calculations that estimate potential soil organic carbon accumulation rates in fields converted from annual to perennial grains of between 0.13 and 1.70 t ha−1 year−1.

Critical comparison of the impact of biochar and wood ash on soil organic matter cycling and grassland productivity

Wood represents the single most important source of renewable energy worldwide and depending on the mechanism of energy production can lead to the production of by-products with vastly different properties (i.e. wood ash (WA) from incineration and biochar (BC) from pyrolysis). These are typically applied to land, however, a critical comparison of their impact on soil quality and carbon (C) cycling is lacking. To address this, we generated biochar (450 °C) and wood ash (870 °C) from the same mixed hardwood feedstock and added it to an agricultural grassland at comparable rates under both laboratory and field conditions (10 t ha−1 and 571 kg ha−1 for BC and WA, respectively). We hypothesized that alkaline, nutrient-rich wood ash would stimulate microbial activity, resulting in the loss of soil organic matter (SOM), while biochar which is recalcitrant to microbial attack would promote the stabilization of native SOM. The effects on the soil microbial community and soil C and N cycling were determined over 1 year. Overall, biochar promoted soil quality by enhancing nutrient availability (P and K), moisture retention and increasing soil C content. However, it was also associated with an increase in below-ground CO2 loss. As plant productivity was unaffected and laboratory incubations of biochar with 14C-labelled SOM showed no indication of priming, we deduce that this CO2 originates from the biochar itself. This is supported by the lack of effect of biochar on soil N cycling, microbial biomass and community structure. Wood ash had almost no effect on either soil quality or vegetation quality (yield and foliar nutrient content) under field conditions but did induce negative SOM priming under both laboratory and field conditions. We conclude that when applied at field-relevant rates, neither amendment had a detrimental effect on native SOM cycling. While wood ash promotes the retention of native SOM, biochar may be a better strategy for enhancing SOM levels because of its intrinsic recalcitrant character, however, this needs to be offset against the reduced amount of energy derived from pyrolysis in comparison to incineration.

Nitrogen fertilization increases rhizodeposit incorporation into microbial biomass and reduces soil organic matter losses

Agricultural soils receive large amounts of anthropogenic nitrogen (N), which directly and indirectly affect soil organic matter (SOM) stocks and CO2 fluxes. However, our current understanding of mechanisms on how N fertilization affects SOM pools of various ages and turnover remains poor. The δ13C values of SOM after wheat (C3)-maize (C4) vegetation change were used to calculate the contribution of C4-derived rhizodeposited C (rhizo-C) and C3-derived SOM pools, i.e., rhizo-C and SOM. Soil (Ap from Haplic Luvisol) sampled from maize rhizosphere was incubated over 56 days with increasing N fertilization (four levels up to 300 kg N ha−1), and CO2 efflux and its δ13C were measured. Nitrogen fertilization decreased CO2 efflux by 27–42% as compared to unfertilized soil. This CO2 decrease was mainly caused by the retardation of SOM (C3) mineralization. Microbial availability of rhizo-C (released by maize roots within 4 weeks) was about 10 times higher than that of SOM (older than 4 weeks). Microbial biomass and dissolved organic C remained at the same level with increasing N. However, N fertilization increased the relative contribution of rhizo-C to microbial biomass by two to five times and to CO2 for about two times. This increased contribution of rhizo-C reflects strongly accelerated microbial biomass turnover by N addition. The decomposition rate of rhizo-C was 3.7 times faster than that of SOM, and it increased additionally by 6.5 times under 300 kg N ha−1 N fertilization. This is the first report estimating the turnover and incorporation of very recent rhizo-C (4 weeks old) into soil C pools and shows that the turnover of rhizo-C was much faster than that of SOM. We conclude that the contribution of rhizo-C to CO2 and to microbial biomass is highly dependent on N fertilization. Despite acceleration of rhizo-C turnover, the increased N fertilization facilitates C sequestration by decreasing SOM decomposition.

Soil and nutrient losses due to root crops harvesting: a case study from southwestern Iran

ABSTRACTAlthough root crops are widely cultivated in Iran, little is known about soil loss due to crop harvesting (SLCH). We assessed the annual exported soil, soil organic matter (SOM) and nutrients from 47 farms under root crops in southwestern Iran. Soil losses for garlic, potato, sugar beet, radish and beetroot were estimated as 6.27, 2.52, 2.26, 4.10 and 6.95, Mg ha−1, respectively, which on average was of the order of soil losses by water erosion in the watershed basins of Iran. Total N, P2O5 and K2O losses were estimated as 36.61, 1.10 and 31.50 kg ha−1 and their costs as 18.24, 0.74 and 19.93 US$ ha−1, respectively. For the whole country, total soil, N, P2O5, K2O and SOM losses for garlic, potato and sugar beet were estimated as 731.7 × 103, 836, 27, 476 and 14.5 × 103 Mg, respectively (radish and beetroot were excluded due to no reliable data on their planted areas). Correlation analysis showed no significant relationship between soil properties and SLCH (except soil moisture content for radish and clay content for beetroot). The findings indicated that the exported soil, SOM and nutrients were at such levels that SLCH should be considered in soil erosion studies.

Thermal alteration of soil organic matter properties: a systematic study to infer response of Sierra Nevada climosequence soils to forest fires

Abstract. Fire is a major driver of soil organic matter (SOM) dynamics, and contemporary global climate change is changing global fire regimes. We conducted laboratory heating experiments on soils from five locations across the western Sierra Nevada climosequence to investigate thermal alteration of SOM properties and determine temperature thresholds for major shifts in SOM properties. Topsoils (0 to 5 cm depth) were exposed to a range of temperatures that are expected during prescribed and wild fires (150, 250, 350, 450, 550, and 650 °C). With increase in temperature, we found that the concentrations of carbon (C) and nitrogen (N) decreased in a similar pattern among all five soils that varied considerably in their original SOM concentrations and mineralogies. Soils were separated into discrete size classes by dry sieving. The C and N concentrations in the larger aggregate size fractions (2–0.25 mm) decreased with an increase in temperature, so that at 450 °C the remaining C and N were almost entirely associated with the smaller aggregate size fractions ( < 0.25 mm). We observed a general trend of 13C enrichment with temperature increase. There was also 15N enrichment with temperature increase, followed by 15N depletion when temperature increased beyond 350 °C. For all the measured variables, the largest physical, chemical, elemental, and isotopic changes occurred at the mid-intensity fire temperatures, i.e., 350 and 450 °C. The magnitude of the observed changes in SOM composition and distribution in three aggregate size classes, as well as the temperature thresholds for critical changes in physical and chemical properties of soils (such as specific surface area, pH, cation exchange capacity), suggest that transformation and loss of SOM are the principal responses in heated soils. Findings from this systematic investigation of soil and SOM response to heating are critical for predicting how soils are likely to be affected by future climate and fire regimes.

Stability of Soil Organic Matter and Soil Loss Dynamics under Short-term Soil Moisture Change Regimes

Soil properties are known to be influenced Soil Organic Matter (SOM) resident time. However, there is limited information on the interactive effects of SOM quality and soil moisture on SOC and Microbial Biomass Carbon (MBC) hence on soil losses. Therefore, this study investigated the effects of different organic matter and soil moisture in soils with low (<2%) initial SOC content on the SOC, MBC and soil loss with time of organic matter incorporation. Six soils were incubated for 34 weeks at 25°C after adding high quality (C/N=23) Vachellia karroo leaf litter and low quality (C/N=41) Zea mays stover. The effect of SOM quality and soil moisture on the SOC content, MBC and soil loss was significantly (P<0.05) the same within but varied across soils. Soils that were continuously wet lost more SOC than under alternating wet-dry moisture conditions. Microbial biomass carbon was controlled by the availability of organic matter and moist soil conditions. Low MBC values corresponded to high SOC and soil loss. Continuously wet soils with high sand particles promoted rapid loss of SOC compared to alternating wet-dry soils. Therefore, continuously wet sandy soils are likely to contribute more to the climate warming than alternating wet-dry soil moisture. In the wake of the climatic change, addition of OM in continuously wet soils need to be regulated but to reduce soil loss re-application of fresh OM has to be more frequent under continuously wet sandy soils than in alternating wet-dry moisture regimes.

Under‐ or Over‐Application of Nitrogen Impact Corn Yield, Quality, Soil, and Environment

Core Ideas Nitrogen application mid‐season cannot overcome drought stress later in the season. Except for crude protein, under‐application of N did not impact forage quality. Soil organic matter decreases in a chisel‐disked corn silage system regardless of N fertilizer rate. Use of compost, cover crops, and conservation tillage can offset soil organic matter losses. Under‐applying N by 30 kg N ha−1 was economically more detrimental than over‐applying. Under‐ or over‐application of N fertilizer to corn (Zea mays L.) has adverse economic and environmental consequences. A 5‐yr study was conducted to determine the impact of N fertilizer on corn silage yield, quality, soil properties, farm economics, and nitrogen‐use efficiency (NUE). Corn silage yields were 12.9, 14.2, and 14.7 Mg ha−1 with most economic rate of nitrogen (MERN) of 90, 95, and 114 kg N ha−1 in 2001, 2003, and 2004 (the three responsive years), respectively. In 2002 and 2005 (non‐responsive years), yields averaged 9.1 Mg ha−1. Yield increased by 3.3 Mg ha−1 with each 10 cm of precipitation in July and August. At the MERN, NUE ranged from 16 (2001) to 25.8 kg DM kg N−1 (2004), reflected in greater soil NO3–N (0–20‐cm depth) at harvest in 2001 as well (23 vs. 8.9 mg kg−1 in 2004). Soil NO3–N at silage harvest in responsive years ranged from 8.9 (2004) to 23 mg kg−1 (2001) in 2001 and in non‐responsive years averaged 22 mg kg−1. Soil NO3–N at harvest was not a good indicator of crop N responsiveness or NUE. Nitrogen addition beyond the MERN decreased NUE and soil pH, and increased crude protein (CP). Under‐application decreased CP and yield in N‐responsive years and increased NUE. Soil organic matter (SOM) was decreased regardless of N rate. Overall, application of 30 kg N ha−1 below the MERN was economically more detrimental than fertilizing the same amount above the MERN.

The Influence of Fire-Induced Surface Changes on the Diurnal Temperature Changes over the Hayman Fire Scar

AbstractDuring the 2010 Bio–Hydro–Atmosphere Interactions of Energy, Aerosols, Carbon, H2O, and Nitrogen (BEACHON) experiment in Colorado, nighttime temperatures over a site within the 2002 “Hayman” fire scar were considerably warmer than over the “Manitou” site that was located outside the fire scar. Temperature differences reached up to 7 K at the surface and extended to an average of 500 m AGL. Afternoon temperatures through the planetary boundary layer (PBL) were similar at the two locations. PBL growth during the day was more rapid at Manitou until 1300 local time, after which the two daytime PBLs had similar temperatures and depths. Observations were taken in fair weather, with weak winds. Runs of the Advanced Research version of the Weather Research and Forecasting model (ARW-WRF) coupled to the Noah-MP land surface model suggest that the fire-induced loss of surface and soil organic matter accounted for the 3–4-K warming at Hayman relative to its prefire state, more than compensating for the cooling due to the fire-induced change in vegetation from forest to grassland. Modeled surface fluxes and soil temperature and moisture changes were consistent with observational studies comparing several-year-old fire scars with adjacent unaffected forests. The remaining difference between the two sites is likely from cold-air pooling at Manitou. It was necessary to increase vertical resolution and replace terrain-following diffusion with horizontal diffusion in ARW-WRF to better capture nighttime near-surface temperature and winds. Daytime PBL growth and afternoon temperature profiles were reasonably reproduced by the basic run with postfire conditions. Winds above the surface were only fairly represented, and refinements made to capture cold pooling degraded daytime temperature profiles slightly.

Experimental warming reveals positive feedbacks to climate change in the Eurasian Steppe.

Identifying soil microbial feedbacks to increasing temperatures and moisture alterations is critical for predicting how terrestrial ecosystems will respond to climate change. We performed a 5-year field experiment manipulating warming, watering and their combination in a semiarid temperate steppe in northern China. Warming stimulated the abundance of genes responsible for degrading recalcitrant soil organic matter (SOM) and reduced SOM content by 13%. Watering, and warming plus watering also increased the abundance of recalcitrant SOM catabolism pathways, but concurrently promoted plant growth and increased labile SOM content, which somewhat offset SOM loss. The treatments also increased microbial biomass, community complexity and metabolic potential for nitrogen and sulfur assimilation. Both microbial and plant community composition shifted with the treatment conditions, and the sample-to-sample compositional variations of the two communities (pairwise β-diversity distances) were significantly correlated. In particular, microbial community composition was substantially correlated with the dominant plant species (~0.54 Spearman correlation coefficient), much more than with measured soil indices, affirming a tight coupling between both biological communities. Collectively, our study revealed the direction and underlying mechanisms of microbial feedbacks to warming and suggested that semiarid regions of northern steppes could act as a net carbon source under increased temperatures, unless precipitation increases concurrently.

Land-use change affects phosphorus fractions in highly weathered tropical soils

Deforestation and land-use change in tropics have increased over the past decades, driven by the demand for agricultural products. Although phosphorus (P) is one of the main limiting nutrients for agricultural productivity in the tropics, the effect of land-use change on P availability remains unclear. The objective was to assess the impacts of land-use change on soil inorganic and organic P fractions of different availability (Hedley sequential fractionation) and on P stocks in highly weathered tropical soils. We compared the P availability under extensive land-use (rubber agroforest) and intensive land-use with moderate fertilization (rubber monoculture plantations) or high fertilization (oil palm monoculture plantations) in Indonesia. The P stock was dominated by inorganic forms (60 to 85%) in all land-use types. Fertilizer application increased easily-available inorganic P (i.e., H2O-Pi, NaHCO3-Pi) in intensive rubber and oil palm plantations compared to rubber agroforest. However, the easily-available organic P (NaHCO3-extractable Po) was reduced by half under oil palm and rubber. The decrease of moderately available and non-available P in monoculture plantation means that fertilization maintains only the short-term soil fertility that is not sustainable in the long run due to the depletion of P reserves. The mechanisms of this P reserve depletion are: 1) soil erosion (here assessed by C/P ratio), 2) mineralization of soil organic matter (SOM) and 3) P export with yield products. Easily-available P fractions (i.e., H2O-Pi, NaHCO3-Pi and Po) and total organic P were strongly positively correlated with carbon content, suggesting that SOM plays a key role in maintaining P availability. Ecologically based management is therefore necessary to mitigate SOM losses and thus increase the sustainability of agricultural production in P-limited, highly weathered tropical soils.

Microbial nitrogen mining affects spatio-temporal patterns of substrate-induced respiration during seven years of bare fallow

Decomposition of soil organic matter (SOM) is regulated by microbial activity, which strongly depends on the availability of carbon (C) and nitrogen (N). Yet, the special role of N on soil organic carbon (SOC) mineralization is still under discussion. The recent concept of microbial N mining predicts increasing SOC mineralization under N-deficiency, which is in contrast to the generally accepted stoichiometric decomposition theory.Following this concept we hypothesized that spatio-temporal patterns of microbial activity are controlled by SOC and N contents, but that microorganisms maintain their functionality to mineralize C under conditions of N deficiency because of microbial N mining.To test this hypothesis, we added glucose to an arable soil that had experienced increasing losses of C3-derived SOM after one, three, and seven years of bare fallow and measured spatio-temporal patterns of substrate-induced respiration (SIR). The SIR measurements were performed with and without additions of mineral N. Selected samples were treated with C4 sugar in order to trace the source of CO2 emissions (sugar vs. SOC-derived) by natural 13C abundance measurements. Sugar additions were repeated after the first SIR experiment to derive information on changing N availability.The results showed that spatial patterns of SIR were not consistently regulated by SOC and N. On a temporal scale, the maximum microbial growth peak declined by 47% from one year bare fallow to seven years bare fallow but soils often developed a second growth phase in the 7th year of fallow. Intriguingly, the maximum microbial growth peak increased again when N was added together with the glucose and no second growth peak occurred. A similar effect was observed after repeated sugar additions but without N additions. The 13C experiment revealed a slightly higher contribution of SOC-derived CO2 in N-deficient samples (16.7%) than in N-fertilized samples (14.6%).We conclude that the first SIR peak was related to the supply of immediately available N while the second growth phase indicated a delayed release of N, due to N mining from SOM. Hence, microbes were able to compensate for initial N limitation and there was no significant change in the overall substrate-induced CO2 release with proceeding time under fallow.

Replacing silage maize for biogas production by sugar beet – A system analysis with ecological and economical approaches

In a holistic methodological approach, we linked field trial data with different modeling approaches to answer the question if sugar beet roots offer an ecological and economical efficient alternative to silage maize as a substrate for biogas production. Field trials were conducted at highly productive sites in Germany, representative for Central Europe, and tested both biomass crops in continuous cultivation and in crop rotations with winter wheat. In these trials, estimated methane yield of silage maize was generally higher (6837 to 8782Nm3ha−1 a−1) than of sugar beet roots (3206 to 7861Nm3ha−1 a−1) and both biomass crops reached highest yield in crop rotations. Under the nonobservance of technical effects, substrate production costs (€ per Nm3 methane) were higher for sugar beet roots and a nationwide modeling showed that, in most of the German districts, it would need to be reduced by 10 to 25% in order to reach economical competitiveness with silage maize. However, at a farm level, sugar beet for biogas production was economically advantageous when introduced with a share of 10 to 16% into the individual farm's cultivation program mainly due to high yield stability reducing the economical risk. However, a decrease in gross margin (€ ha−1) was likely to occur. In the field trials, different ecological impacts of crop cultivation were assessed but did not highlight one of the two biomass crops in comparison. However, it was evident that cultivating them in three years long crop rotations with two years of winter wheat provoked lower risks of loss of soil organic matter (−122 to −20kg humus-C ha−1 a−1) or N-leaching (40 to 62kgNha−1 in three years) than in continuous cultivation. In contrast, the continuous cultivation of silage maize and sugar beet showed lower greenhouse gas emission (7652 to 11,074kg C-dioxide-equivalents ha−1 in three years) than the crop rotations with winter wheat. Overall, we conclude that sugar beet roots can offer an efficient alternative to silage maize as a substrate for biogas production. However, to raise sugar beet's competitiveness, dry matter yields should be increased without increasing production costs and ecological impacts.

Carbon Fluxes from Different Pools in a Mined Area under Reclamation in Minas Gerais State, Brazil

AbstractDespite the benefits of grass cultivation and organic fertilization in mining areas undergoing reclamation, these practices may be associated to CO2 emissions and soil organic matter (SOM) losses by priming effect. In the present study, we evaluated the changes on SOM pools and C–CO2 emissions in a bauxite‐mined area under reclamation fertilized with poultry litter (PL) (0, 10, 20, and 40 Mg ha−1) and cultivated with Brachiaria brizantha. Increases of about 3·5 times in the soil labile C were observed 1 year after experiment establishment. High C–CO2 fluxes and a significant positive priming effect were observed in the presence of B. brizantha, increasing native C mineralization by nearly 4·9 times. Nevertheless, no net soil C loss was detected, probably because of the C inputs derived from B. brizantha, which offset these losses. In fact, the grass increased total organic C by 45% when fertilized with 40 Mg PL ha−1. The data obtained suggest that the cultivation of B. brizantha fertilized with PL can be a promising option for rapid recovery in SOM in areas under reclamation. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Spatial patterns of soil resources under different land use in Prosopis woodlands of the Monte desert

Changes in the spatial distribution of resources constitute an indicator of degradation of arid grazing lands. In arid and semi-arid ecosystems, the distribution of soil resources has been commonly associated with the structure and the spatial arrangements of the vegetation. Although the formation of “fertile islands” beneath vegetation patches is well documented, much less is known about the changes induced by grazing systems on the distribution of soil resources. We examine how pastoralist settlements are affecting the spatial distribution of soil resources and the soil nutrient balance in central-western woodlands of Argentina. We analyzed the distribution of soil water, chloride, nitrate, total nitrogen, and organic matter at increasing distances from livestock corrals and in undisturbed woodlands, at different soil depths. We also calculated variation indexes of soil organic matter and total nitrogen produced by livestock settlements, as an indicator of degree of deterioration or improvement of the soils. The transects located in pastoralist settlements demonstrated an increasing centripetal gradient in availability of soil water and nutrients compared to transects outside of these disturbed areas. Livestock corrals create local hotspots of nutrient enrichment, but when we analyzed the effects of livestock settlements at a higher spatial scale, we found net losses of soil organic matter and total nitrogen. We conclude that the coupling between nutrient and patch dynamics is disrupted by the pastoralist settlements, which caused a redistribution of soil resources, controlled by the location from the livestock corrals. The processes that promote nutrient losses, such as ammonium volatilization, denitrification, nitrate leaching, organic matter oxidation, manure exports, and soil erosion, are relatively higher than the extra inputs of dung and urine. Therefore, this study emphasizes the role of grazing systems as modulators of water and nutrient fluxes, and soil nutrient balance.

Improving Farming Practices for Sustainable Soil Use in the Humid Tropics and Rainforest Ecosystem Health

Unsustainable farming practices such as shifting cultivation and slash-and-burn agriculture in the humid tropics threaten the preservation of the rainforest and the health of the local and global environment. In weathered soils prone to cohesion in humid tropic due to low Fe and carbon content and the enormous amounts of P that can be adsorbed, sustainable soil use is heavily dependent on the availability and efficient use of nutrients. This paper reviews the literature in the field and provides some insights about sustainable soil use in the humid tropics, mainly for the Brazilian Amazonia region. Careful management of organic matter and physical and chemical indicators is necessary to enhance root growth and nutrient uptake. To improve the rootability of the arable layer, a combination of gypsum with continuous mulching to increase the labile organic matter fraction responsible for the formation of a short-lived structure important for root growth is recommended, rather than tillage. Unlike mulching, mechanical disturbance via ploughing of Amazonian soils causes very rapid and permanent soil organic matter losses and often results in permanent recompaction and land degradation or anthropic savannization; thus, it should be avoided. Unlike in other regions, like southeast Brazil, saturating the soil solely with inorganic potassium and nitrogen soluble fertilizers is not recommended. Nutrient retention in the root zone can be enhanced if nutrients are added in a slow-release form and if biologically mediated processes are used for nutrient release, as occurs in green manure. Therefore, an alternative that favors using local resources to increase the supply of nutrients and offset processes that impair the efficiency of nutrient use must be pursued.

Cover crops have neutral effects on predator communities and biological control services in annual cellulosic bioenergy cropping systems

Maize stover is beginning to be used as a cellulosic biofuel feedstock in the Midwestern United States; however, there are concerns that stover removal could result in increased soil erosion and loss of soil organic matter. Use of a winter cover crop following maize harvest has the potential to mitigate these impacts and may have additional benefits by providing continuous living cover in annual crop habitats leading to changes in insect predator communities and increased biocontrol services. However, cover crops may also be harvested in cellulosic biofuel systems, adding a disturbance event that may negatively affect biocontrol. We contrasted insect predator communities and their impacts in four potential bioenergy cropping systems in Michigan and Wisconsin (USA) during the 2013 and 2014 growing seasons. Two annual maize systems were harvested for grain and stover; one maize system included a cereal rye/Austrian winter pea cover crop. Two perennial systems, switchgrass and a mixed prairie grasses and forbs, were harvested in the fall for biomass. Predatory insect abundance and diversity were lower in both annual cropping systems than in the perennial cropping systems and the inclusion of the cover crop did not significantly alter these responses. Similarly, removal of sentinel insect egg prey was also lower in the annual versus perennial cropping systems, with no significant influence of cover crop. We also explored the potential for cover crops to harbor prey populations in the spring that might encourage oviposition by mobile predators and potentially lead to local population sources or sinks of predators depending on the timing and effect of cover crop harvest. We found that existing predator communities in the cover crop treatments effectively suppressed aphid population growth, limiting their attractiveness to mobile predators. While we found no significant positive or negative impacts of this cover crop system on biocontrol services, bioenergy cover cropping systems could be managed to increase multiple ecosystem services by altering cover crop identity, or timing of planting and harvest.

Climate change amplifies gross nitrogen turnover in montane grasslands of Central Europe in both summer and winter seasons.

The carbon- and nitrogen-rich soils of montane grasslands are exposed to above-average warming and to altered precipitation patterns as a result of global change. To investigate the consequences of climatic change for soil nitrogen turnover, we translocated intact plant-soil mesocosms along an elevational gradient, resulting in an increase of the mean annual temperature by approx. 2°C while decreasing precipitation from approx. 1500 to 1000mm. Following three years of equilibration, we monitored the dynamics of gross nitrogen turnover and ammonia-oxidizing bacteria (AOB) and archaea (AOA) in soils over an entire year. Gross nitrogen turnover and gene levels of AOB and AOA showed pronounced seasonal dynamics. Both summer and winter periods equally contributed to cumulative annual N turnover. However, highest gross N turnover and abundance of ammonia oxidizers were observed in frozen soil of the climate change site, likely due to physical liberation of organic substrates and their rapid turnover in the unfrozen soil water film. This effect was not observed at the control site, where soil freezing did not occur due to a significant insulating snowpack. Climate change conditions accelerated gross nitrogen mineralization by 250% on average. Increased N mineralization significantly stimulated gross nitrification by AOB rather than by AOA. However, climate change impacts were restricted to the 2-6cm topsoil and rarely occurred at 12-16cm depth, where generally much lower N turnover was observed. Our study shows that significant mineralization pulses occur under changing climate, which is likely to result in soil organic matter losses with their associated negative impacts on key soil functions. We also show that N cycling processes in frozen soil can be hot moments for N turnover and thus are of paramount importance for understanding seasonal patterns, annual sum of N turnover and possible climate change feedbacks.

Soil organic matter and nutrient accumulation in areas under intensive management and swine manure application

Land use change and soil management are frequently associated to land degradation and soil organic matter (SOM) losses in tropical regions. In Brazil, in order to avoid this process, different management strategies have been applied, such as no-tillage and agricultural disposal of swine manure (SM). This study was carried out to evaluate the quantity and quality of SOM, as well as the occurrence of nutrient accumulation in soils of areas under contrasting management systems that have received consecutive applications of SM over the last decades in Brazil. Five land uses were sampled: native vegetation (NV), pasture with SM application (PA + SM), no-tillage with SM application (NT + SM), no-tillage (NT), and conventional tillage with SM application (CT + SM). Soil organic carbon (SOC), N, labile C, C management index (CMI), P, Ca2+, Mg2+, K+, Al3+, Fe, Zn, Mn, Cu, and H + Al were quantified. Except for PA + SM, the agricultural land uses caused decreases in SOC contents comparing to NV. PA + SM showed the highest C stocks, 138.9 ± 3.4 Mg ha−1 down to 0.4 m. The application of SM can be associated to the greater C stocks in PA + SM, NT + SM, and CT + SM and to the higher N contents in all land uses under this practice. Land uses which receive higher rates of swine manure application (PA + SM and CT + SM) have shown CMI greater than 100. However, this practice is associated to the accumulation of P, Cu, Na, and Zn in these soils. The SM application is associated to improvement on C stocks and SOM quality in area under pasture, no-tillage, and conventional tillage in Parana State, Brazil. However, this practice is the main driver of nutrient accumulation in these areas.

Tillage and crop rotation effects on carbon sequestration and aggregate stability in two contrasting soils at the Zanyokwe Irrigation Scheme, Eastern Cape province, South Africa

Intensive tillage and monocropping have adversely affected the quality of soils in South Africa through accelerated loss of soil organic matter. Two clay loam soils, a Bonheim at Burnshill and a Shortlands at Lenye, at the Zanyokwe Irrigation Scheme in the Eastern Cape province were used to evaluate the short-term effects of tillage and crop rotations on carbon sequestration and aggregate stability under sprinkler-irrigated crop production. A split-plot arrangement of treatments in a randomised complete block design was used with tillage as the main plots and crop rotations as subplots. Conventional tillage (CT) was compared to no-till (NT) under maize–fallow–maize (MFM), maize– wheat–maize (MWM) and maize–oat–maize (MOM) rotations. Carbon sequestration was monitored by measuring changes in soil organic carbon (SOC) and the stability index (SI) was used for monitoring aggregate stability. No-till had inconsistent effects on SOC relative to CT but resulted in improved soil SI on both soils, especially on the Shortlands soil. The MOM rotation enhanced SOC relative to the MWM and MFM rotations on both the Bonheim and Shortlands soils. Across tillage practices, the MOM rotation significantly increased the soil aggregate SI compared with the MWM and MFM rotations on the Shortlands and to a lesser extent on the Bonheim soils. Scanning electron microscopy revealed that soil aggregates under MOM had dense organic coatings and bridges compared with the MFM rotation, indicating the positive effect of carbon sequestration on aggregate stability. Generally, the results indicated that, in the short term, cover crops, especially oats, have greater influence on SOC accumulation and aggregate stability than tillage, irrespective of soil type.

Evidence for Losses From Strongly Bound SOM Pools After Clear Cutting in a Northern Hardwood Forest

Forest soils in the northeastern United States store considerable amounts of carbon (C). With the increasing utilization of biomass as a “C-neutral” form of energy in the United States, these forests are susceptible to clear cutting and large losses of soil organic matter (SOM) to the atmosphere as carbon dioxide (CO2). The relative stability versus susceptibility of SOM to degradation can be approximated, in part, through the strength of organo-mineral interactions, that is, the strength of binding between SOM and mineral surfaces in the soil. This study investigated differences in SOM organo-mineral binding between northern hardwood forest stands with varying clear-cutting histories in Bartlett Experimental Forest in Bartlett, New Hampshire. Sequential chemical extractions were performed to quantify SOM storage in organo-mineral pools of various binding strength. In this case study, soils from Mature forest stands stored significantly more SOM in strongly mineral-bound and stable C pools than soils from Cut stands did. Differences in the relative distribution of C in organo-mineral pools in Mature and Cut forests may inform our understanding of SOM bioavailability, microbial decomposition, and CO2 production in ecosystems after clear cutting. These findings should contribute to discussions on long-term SOM stability in northeastern U.S. soils.