Abstract. Physical fractionation is a widely used methodology to study soil organic matter (SOM) dynamics, but concerns have been raised that the available fractionation methods do not well describe functional SOM pools. In this study we explore whether physical fractionation techniques isolate soil compartments in a meaningful and functionally relevant way for the investigation of litter-derived nitrogen dynamics at the decadal timescale. We do so by performing aggregate density fractionation (ADF) and particle size-density fractionation (PSDF) on mineral soil samples from two European beech forests a decade after application of 15N labelled litter. Both density and size-based fractionation methods suggested that litter-derived nitrogen became increasingly associated with the mineral phase as decomposition progressed, within aggregates and onto mineral surfaces. However, scientists investigating specific aspects of litter-derived nitrogen dynamics are pointed towards ADF when adsorption and aggregation processes are of interest, whereas PSDF is the superior tool to research the fate of particulate organic matter (POM). Some methodological caveats were observed mainly for the PSDF procedure, the most important one being that fine fractions isolated after sonication can not be linked to any defined decomposition pathway or protective mechanism. This also implies that historical assumptions about the "adsorbed" state of carbon associated with fine fractions need to be re-evaluated. Finally, this work demonstrates that establishing a comprehensive picture of whole soil OM dynamics requires a combination of both methodologies and we offer a suggestion for an efficient combination of the density and size-based approaches.

Density fractions versus size separates: Does physical fractionation isolate functional soil compartments?

Organic amendments, such as compost and biochar, mitigate the environmental burdens associated with wasting organic resources and close nutrient loops by capturing, transforming, and resupplying nutrients to soils. While compost or biochar application to soil can enhance an agroecosystem's capacity to store carbon and produce food, there have been few field studies investigating the agroecological impacts of amending soil with biochar co‐compost, produced through the composting of nitrogen‐rich organic material, such as manure, with carbon‐rich biochar. Here, we examine the impact of biochar co‐compost on soil properties and processes by conducting a field study in which we compare the environmental and agronomic impacts associated with the amendment of either dairy manure co‐composted with biochar, dairy manure compost, or biochar to soils in a winter wheat cropping system. Organic amendments were applied at equivalent C rates (8 Mg C ha−1). We found that all three treatments significantly increased soil water holding capacity and total plant biomass relative to the no‐amendment control. Soils amended with biochar or biochar co‐compost resulted in significantly less greenhouse gas emissions than the compost or control soils. Biochar co‐compost also resulted in a significant reduction in nutrient leaching relative to the application of biochar alone or compost alone. Our results suggest that biochar co‐composting could optimize organic resource recycling for climate change mitigation and agricultural productivity while minimizing nutrient losses from agroecosystems.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1111/gcbb.13028

Biochar co‐compost improves nitrogen retention and reduces carbon emissions in a winter wheat cropping system

Abstract. We studied the long-term impact of contrasting tillage
and cover cropping systems on soil structure and hydraulic properties.
Complete water retention and conductivity curves for the top (0–5 cm) and
subsurface (20–25 cm) soils were characterized and contrasted. Dynamic
water storage and retention were evaluated using numerical simulations in
HYDRUS-2D software. Compared with standard-till (ST) and no-cover-crop (NO)
systems, soils under no-till (NT) and cover cropping (CC) systems showed
improved soil structure in terms of pore size distribution (PSD). Changes in
hydraulic conductivity (K) under these systems led to an increased infiltration
rate and water retention. However, NT and CC plots had lower water content
at field capacity (33 kPa suction) and lower plant-available water (PAW)
compared with ST and NO plots. Numerical simulations, however, showed that NT
and CC plots have higher water storage (albeit marginal in magnitude) and
water availability following irrigation. Because the numerical simulations
considered retention and conductivity functions simultaneously and
dynamically through time, they allow the capture of hydraulic states that
are arguably more relevant to crops. The study concludes that the long-term
practices of NT and CC systems were beneficial in terms of changes to the
PSD. NT and CC systems also marginally improved soil water
conductivity and storage at the plot scale.


/pdf/long-term-impact-of-cover-crop-and-reduced-disturbance-3hvg3wqc.pdf

Long-term impact of cover crop and reduced disturbance tillage on soil pore size distribution and soil water storage

Soil organic carbon (SOC) enrichment in aeolian sediments and SOC loss by dust emission in the desert steppe, China.

Carbon footprint and net carbon gain of major long-term cropping systems under no-tillage.

Increasing soil organic carbon (SOC) stocks in agricultural soils can contribute to stabilizing or even lowering atmospheric greenhouse gas (GHG) concentrations. Cover crop rotation has been shown to increase SOC and provide productivity benefits for agriculture. Here we used a split field design to evaluate the short-term effect of cover crop on SOC distribution and chemistry using a combination of bulk, isotopic, and spectroscopic analyses of size-and density-separated soil aggregates. Macroaggregates (>250 µm) incorporated additional plant material with cover crop as evidenced by more negative δ13C values (−25.4‰ with cover crop compared to −25.1‰ without cover crop) and increased phenolic (plant-like) resonance in carbon NEXAFS spectra. Iron EXAFS data showed that the Fe pool was composed of 17–21% Fe oxide with the remainder a mix of primary and secondary minerals. Comparison of oxalate and dithionite extractions suggests that cover crop may also increase Fe oxide crystallinity, especially in the dense (>2.4 g cm−3) soil fraction. Cover crop δ13C values were more negative across density fractions of bulk soil, indicating the presence of less processed organic carbon. Although no significant difference was observed in bulk SOC on a mass per mass basis between cover and no cover crop fields after one season, isotopic and spectroscopic data reveal enhanced carbon movement between aggregates in cover crop soil.

Effect of Cover Crop on Carbon Distribution in Size and Density Separated Soil Aggregates

The advent of “natural climate solutions” and “climate smart agriculture” has increased interest in managing agricultural lands to sequester soil carbon and mitigate climate change. This has led to enormous opportunities for soil scientists and growers alike, as new soil carbon initiatives are created by public, private, and philanthropic entities. It has also led to confusion over what is possible or practical to achieve through agricultural management, as soil carbon formation and storage is complex, and its response to management is context-dependent. This can pose challenges to decision makers tasked with creating defensible, science-informed policies and programs for building and protecting soil carbon. Here we summarize the science concerning the potential for agricultural soils to serve as a natural climate solution, in order to frame a discussion of current approaches in United States (US) policy and practice. We examine existing strategies such as soil health initiatives and direct incentive payments, as well as emerging schemes such as carbon markets and crop insurance reform. We suggest future directions for each strategy, and make recommendations for synthesizing approaches into a cohesive US policy portfolio. Guiding principles for this discussion include the notions that (i) climate change adaptation must be prioritized alongside climate change mitigation; (ii) soil carbon sequestration must be paired with greenhouse gas emission reductions; (iii) structural issues and barriers to adoption must be addressed as part of all policies and programs; (iv) practice- and place-specific programs must be administered in lieu of one-size-fits-all prescriptions; and (v) soil carbon science is not yet sufficiently advanced for the accounting and contractual frameworks proposed in cap-and-trade or regulatory approaches.

/pdf/grounding-united-states-policies-and-programs-in-soil-carbon-1u6w4vku.pdf

Grounding United States policies and programs in soil carbon science: strengths, limitations, and opportunities

. Subsurface carbon stocks are a prime target for efforts to increase soil carbon storage for climate change mitigation and improving soil health. However, subsurface carbon (C) dynamics are not well understood, especially in soils under long term intensive agricultural management. We compared subsurface C dynamics in tomato-corn rotations after 25 years of differing C and nutrient management in the California Central Valley: CONV (mineral fertilizer), CONV+WCC (mineral fertilizer + cover crops) and ORG (composted poultry manure + cover crops). Our results showed a ~19 Mg/ha increase in SOC stocks down to 1 m under ORG systems, no significant SOC increases under CONV+WCC or CONV systems, and the accumulation of carboxyl rich C in the subsurface (60–100 cm) horizons of all systems. Systems also had greater amounts of aromatic carbon in the order ORG>CONV+WCC>CONV. We identified a potential interaction between cover crops and compost, theorizing that increased macropores from cover crop roots facilitate the transport of soluble C and nutrients into the subsurface, thereby increasing stocks. These results demonstrate the potential for subsurface carbon storage in tilled agricultural systems and highlight a potential pathway for increasing carbon transport and storage in subsurface soil layers.

/pdf/synergy-between-compost-and-cover-crops-leads-to-2d10h27c2y.pdf

Synergy between compost and cover crops leads to increasedsubsurface soil carbon storage

Abstract. Subsoil carbon (C) stocks are a prime target for efforts to increase soil C storage for climate change mitigation. However, subsoil C dynamics are not well understood, especially in soils under long-term intensive agricultural management. We compared subsoil C storage and soil organic matter (SOM) composition in tomato–corn rotations after 25 years of differing C and nutrient management in the California Central Valley: CONV (mineral fertilizer), CONV+WCC (mineral fertilizer and cover crops), and ORG (composted poultry manure and cover crops). The cover crop mix used in these systems is a mix of oat (Avena sativa L.), faba bean (Vicia faba L.), and hairy vetch (Vicia villosa Roth). Our results showed a ∼19 Mg ha−1 increase in soil organic C (SOC) stocks down to 1 m under ORG systems, no significant SOC increases under CONV+WCC or CONV systems, and an increased abundance of carboxyl-rich C in the subsoil (60–100 cm) horizons of ORG and CONV+WCC systems. Our results show the potential for increased subsoil C storage with compost and cover crop amendments in tilled agricultural systems and identify potential pathways for increasing C transport and storage in subsoil layers.


/pdf/synergy-between-compost-and-cover-crops-in-a-mediterranean-3eo03fmi.pdf

Synergy between compost and cover crops in a Mediterranean row crop system leads to increased subsoil carbon storage

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Effect of Cover Crop on Carbon Distribution in Size and Density Separated Soil Aggregates

Grounding United States policies and programs in soil carbon science: strengths, limitations, and opportunities

Synergy between compost and cover crops leads to increasedsubsurface soil carbon storage

Synergy between compost and cover crops in a Mediterranean row crop system leads to increased subsoil carbon storage