Items

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  A simple soil organic carbon level metric beyond the organic carbon-to-clay ratio

  Soil is a precious and non-renewable resource that is under increasing pressure and the development of indicators to monitor its state is pivotal. Soil organic carbon (SOC) is important for key physical, chemical and biological soil properties and thus a central indicator of soil quality and soil health. The content of SOC is driven by many abiotic factors, such as texture and climate, and is therefore strongly site-specific, which complicates, for example, the search for appropriate threshold values to differentiate healthy from less healthy soils. The SOC:clay ratio has been introduced as a normalized SOC level metric to indicate soils' structural condition, with classes ranging from degraded (<1:13) to very good (>1:8). This study applied the ratio to 2958 topsoils (0–30 cm) in the German Agricultural Soil Inventory and showed that it is not a suitable SOC level metric since strongly biased, misleading and partly insensitive to SOC changes. The proportion of soils with SOC levels classified as degraded increased exponentially with clay content, indicating the indicator's overly strong clay dependence. Thus, 94% of all Chernozems, which are known to have elevated SOC contents and a favourable soil structure, were found to have either degraded (61%) or moderate (33%) normalized SOC levels. The ratio between actual and expected SOC (SOC:SOCexp) is proposed as an easy-to-use alternative where expected SOC is derived from a regression between SOC and clay content. This ratio allows a simple but unbiased estimate of the clay-normalized SOC level. The quartiles of this ratio were used to derive threshold values to divide the dataset into the classes degraded, moderate, good and very good. These classes were clearly linked to bulk volume (inverse of bulk density) as an important structural parameter, which was not the case for classes based on the SOC:clay ratio. Therefore, SOC:SOCexp and its temporal dynamic are proposed for limited areas such as regions, states or pedoclimatic zones, for example, in a soil health monitoring context; further testing is, however, recommended.
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  Maximizing soil organic carbon stocks under cover cropping: insights from long-term agricultural experiments in North America

  Cover crops are widely advocated for increasing soil organic carbon (SOC) levels, thereby benefiting soil health improvement and climate change mitigation. Few regional-scale studies have robustly explored SOC stocks under cover cropping, due to limited long-term experiments. We used the unique experimental data from the North American Project to Evaluate Soil Health Measurements conducted in 2019 to address this issue. This study included 19 agricultural research sites with 36 pairs of cover cropping established between 1896 and 2014. Explanatory variables related to site-specific environmental conditions and management practices were collected to identify and prioritize contributing factors that affect SOC stocks with cover crops, by coupling the Boruta algorithm and structural equation modeling. Overall, cover crops significantly (P < 0.05) improved several indicators of soil health, including greater SOC (concentration: +8%; stock: +7%), total nitrogen (+8%), water-stable aggregates (+15%), and potential carbon mineralization (+34%), on average, compared to no cover crop control. Likewise, on average, cover crops sequestered SOC 3.55 Mg C ha-1 (0–15 cm depth), with a sequestration rate of 0.24 Mg C ha-1 yr-1. In addition, we found climate (Hargreaves climatic moisture deficit) was important in explaining the variation of SOC stocks with cover crops, followed by soil properties (e.g., soil clay content). In terms of management practices, cover crop type had a significant positive (0.36) effect on SOC stocks, with non-legumes showing a greater impact, compared to legumes and mixtures. Crop rotational diversity also had a positive (0.28) effect on SOC accumulation. Our findings suggested that integrating non-legume cover crops into diverse crop rotation is likely to be a promising strategy to maximize SOC stocks with cover crops across North America.
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  Initial soil organic carbon stocks govern changes in soil carbon: Reality or artifact?

  Changes in soil organic carbon (SOC) storage have the potential to affect global climate; hence identifying environments with a high capacity to gain or lose SOC is of broad interest. Many cross-site studies have found that SOC-poor soils tend to gain or retain carbon more readily than SOC-rich soils. While this pattern may partly reflect reality, here we argue that it can also be created by a pair of statistical artifacts. First, soils that appear SOC-poor purely due to random variation will tend to yield more moderate SOC estimates upon resampling and hence will appear to accrue or retain more SOC than SOC-rich soils. This phenomenon is an example of regression to the mean. Second, normalized metrics of SOC change—such as relative rates and response ratios—will by definition show larger changes in SOC at lower initial SOC levels, even when the absolute change in SOC does not depend on initial SOC. These two artifacts create an exaggerated impression that initial SOC stocks are a major control on SOC dynamics. To address this problem, we recommend applying statistical corrections to eliminate the effect of regression to the mean, and avoiding normalized metrics when testing relationships between SOC change and initial SOC. Careful consideration of these issues in future cross-site studies will support clearer scientific inference that can better inform environmental management.
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  The nitrogen gap in soil health concepts and fertility measurements

  Soil nitrogen (N) often limits productivity in agroecosystems, prompting fertilizer applications that increase crop yields but can degrade the environment. Nitrogen's dual role in both productivity and environmental quality should center it in soil health frameworks. We use recent evidence to argue that N availability is an emergent property of the integrated soil biogeochemical system and is strongly influenced by plant traits and their interactions with microbes and minerals. Building upon this, we theorize that the sources of plant and microbial N shift across soil health gradients, from inorganic N dependence in ecologically simple systems with poor soil health to a highly networked supply of organic N in healthy soils; ergo, investments in soil health should increase ecological complexity and the pathways by which plants can access N, leading to more resilient nutrient supplies and yields in a variable climate. However, current N assessment methods derive from a historical emphasis on inorganic N pool sizes and are unable to capture the shifting drivers of N availability across soil health gradients. We highlight the need to better understand the plant-microbial-mineral interactions that regulate bioavailable N as a first step to improving our ability to measure it. We conclude it will be necessary to harness agroecosystem complexity, account for plant and microbial drivers, and carefully integrate external N inputs into soils' internal N network to expand the routes by which N from organic pools can be made bioavailable. By emphasizing N in soil health concepts, we argue that researchers can accelerate advances in N use efficiency and resiliency.
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  Chapter Three - Soil carbon accumulation in crop-livestock systems in acid soil savannas of South America: A review

  Acid soil savannas of tropical America are a vast resource to expand agricultural production, alleviate the pressure on tropical rainforest and reduce greenhouse gas (GHG) emissions. During the past three decades there have been major changes in land use in the Cerrados of Brazil and to a lesser extent in the Llanos of Colombia. Monocropping and improved pasture grasses were adopted widely to boost crop and animal production. Various types of integrated crop-livestock systems and no-till cropping systems were introduced to not only recuperate degraded pastures but also to sustain crop and livestock productivity. Several studies showed that well-managed pastures based on deep rooted tropical forage grass and legume species could accumulate significant amounts of soil organic carbon (SOC) in deeper soil layers. Among the number of factors that influence SOC accumulation, deep rooting ability of grasses and high root turnover seem to play a major role in accumulation of SOC in deeper soil layers in the form of particulate organic carbon (POC) and mineral associated organic carbon (MAOC). This review provides insights toward some key approaches and management options to increase both POC and MAOC accumulation and particularly MAOC accumulation in deeper soil layers in crop-livestock systems. There are some important gaps in our knowledge, particularly regarding the influence of length of pasture phase on MAOC accumulation in deeper soil layers from crop-livestock systems. Finally, we highlight the importance of land use policies and suggest some future research priorities for consideration to increase benefits from the use of integrated crop-livestock systems in acid soil savannas.
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  The Science and Semantics of “Soil Organic Matter Stabilization”

  It is a shared goal of the scientific community to improve model projections of soil organic carbon dynamics such that the carbon cycle is represented as accurately as possible in global land models. For several decades, those pursuing this objective have operated under the assumption that “the longer soil carbon persists, the more stable we regard it as being.” While this understanding was helpful in facilitating conversations on the topic, it has created the notion of the existence of a chemically and physically definable organic phase in soil with the ability to “resist” decomposition. Here we argue that “stability” is a conceptual and not a physically existing state of organic matter. To this end, we analyze the semantics of “persistence” and “stability” to point out that (i) they are independent concepts and not interchangeable terms and (ii) that “persistence” is not an automatic consequence of “stability.” Through extensive revision of the literature, we show that slow-cycling carbon does not do so because it has the material property of being “stable,” rather, it persists because it is not being decomposed. It follows that the notion of soil organic matter stabilization is a flawed concept that distracts from the actual causes for slow carbon cycling. We suggest that the question to be asked in future research should no longer be “Why is soil carbon stable?.” Rather, the question should be: What are the constraints that prevent the decomposer community from processing soil carbon to their full metabolic potential?
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  Roots are key to increasing the mean residence time of organic carbon entering temperate agricultural soils

  The ratio of soil organic carbon stock (SOC) to annual carbon input gives an estimate of the mean residence time of organic carbon that enters the soil (MRTOC). It indicates how efficiently biomass can be transformed into SOC, which is of particular relevance for mitigating climate change by means of SOC storage. There have been few comprehensive studies of MRTOC and their drivers, and these have mainly been restricted to the global scale, on which climatic drivers dominate. This study used the unique combination of regional-scale cropland and grassland topsoil (0–30 cm) SOC stock data and average site-specific OC input data derived from the German Agricultural Soil Inventory to elucidate the main drivers of MRTOC. Explanatory variables related to OC input composition and other soil-forming factors were used to explain the variability in MRTOC by means of a machine-learning approach. On average, OC entering German agricultural topsoils had an MRT of 21.5 ± 11.6 years, with grasslands (29.0 ± 11.2 years, n = 465) having significantly higher MRTOC than croplands (19.4 ± 10.7, n = 1635). This was explained by the higher proportion of root-derived OC inputs in grassland soils, which was the most important variable for explaining MRTOC variability at a regional scale. Soil properties such as clay content, soil group, C:N ratio and groundwater level were also important, indicating that MRTOC is driven by a combination of site properties and OC input composition. However, the great importance of root-derived OC inputs indicated that MRTOC can be actively managed, with maximization of root biomass input to the soil being a straightforward means to extend the time that assimilated C remains in the soil and consequently also increase SOC stocks.
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  Carbon and nitrogen transfer from litter to soil is higher in slow than rapid decomposing plant litter: A synthesis of stable isotope studies

  Litter decomposability determines litter mass loss rate, but how it affects soil carbon (C) and nitrogen (N) storage remains elusive. We compiled data from 25 litter decomposition studies tracing the fate of C and N during decomposition using stable C and N isotopes. An average of 24% of C lost from decomposing litter was recovered in the soil independent of the decomposition stage and the experimental conditions. In contrast, a higher amount of N lost from decomposing litter was recovered in the soil in laboratory (80%) than in field (58%) experiments. The proportion of the total C and N lost that was transferred to the soil was higher for slowly than for rapidly decomposing litter types. Our results demonstrate substantial soil C and especially N input from decomposing litters and suggest that slowly decomposing litters favor soil C and N storage compared to more rapidly decomposing litters.
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  Priming mechanisms providing plants and microbes access to mineral-associated organic matter

  Mineral-associated organic matter (MAOM) is considered a stable reservoir for soil nutrients that influences long-term soil carbon (C) and nitrogen (N) dynamics. However, recent experimental and theoretical evidence shows that root exudates may mobilize MAOM, thereby providing plants and microbes access to a large and N-rich pool. Given the mechanisms underlying MAOM C and N mobilization remain largely untested, we examined direct and indirect pathways by which root exudates destabilize this nutrient pool in laboratory mesocosms. We simulated root exudation with 13C-labeled oxalic acid to test whether root exudates are directly capable of mobilizing MAOM from mineral surfaces; and with 13C-labeled glucose to test whether indirect stimulation of microbial and extracellular enzyme activity leads to MAOM decomposition. We also tested the potential for oxalic acid and glucose to mobilize MAOM in an additional subset of sterilized soils to clarify the potential for non-microbial pathways of MAOM destabilization. Over the course of the 12-day MAOM incubation with and without simulated exudates, we measured C cycling (CO2 respiration rates, 13C–CO2 efflux), N cycling (inorganic N pools, gross N mineralization) and related microbial processes (enzyme activities and microbial community composition via phospholipid fatty acid analysis). Both of the simulated root exudates enhanced MAOM-C mineralization, with cumulative respiration increasing 35–89% relative to the water-only control. Likewise, glucose additions enhanced the production of an exo-cellulase and a chitinase by up to 130% and 39%, respectively, while oxalic acid enhanced oxidative enzyme activities up to 91% greater than control rates. We observed a positive association between glucose-induced shifts in enzyme activities, MAOM-C mineralization, and gross ammonification. Oxalic acid additions were associated with initial increases in fungal relative abundance and in sterile soils appeared to stimulate the release of metals and dissolved organic nitrogen into exchangeable pools. Our results indicate that common root exudates, like glucose and oxalic acid, can significantly increase the turnover and potential release of C and N from MAOM through indirect (e.g., enzyme induction) and direct (e.g., mobilization of metal oxides) mechanisms.