•Reduced tillage, residue retention increased C and N versus conventional management.•Enzyme activity increased with conservation management.•Conservation management increased enzyme activities in ...all particle-size fractions.•Magnitude of increases greater for enzyme activities than C and N stocks.•Enzyme activity sensitive indicator of soil health in response to management change.
Soil organic matter (SOM) concentration and enzyme activity are important biochemical indicators of soil health for assessing the sustainability of agricultural management practices. However, little is known about the long-term effects of tillage and crop residue management on SOM and enzyme activities in soil particle-size fractions on the Loess Plateau of Northern China. The objective of this study was to investigate the effects of 11 years of combined tillage and crop residue management treatments on soil organic carbon (SOC), total nitrogen (TN) concentrations and enzyme activities in bulk soil and particle-size fractions from a rainfed wheat (Triticum aestivum L.) monoculture system in this region. We hypothesized that reduced tillage and increased residue retention would increase SOC, TN and enzyme activities in both bulk soil and particle-size fractions, and that enzyme activity would serve as a more sensitive indicator of soil health in response to management. Compared with conventional tillage and residue removal (CTRR), reduced tillage and stubble mulch residue retention (RTSM) increased bulk soil activities of most enzymes (sulfatase +68%, invertase +62%, β-glucosidase +58%, dehydrogenase +46%). These increases were greater than the relative increases in total SOC (34%) and TN (33%) concentrations, supporting our hypothesis of a stronger response in microbial activity to management than total element stocks. The RTSM treatment also increased SOC and TN concentrations, as well as β-glucosidase, acid phosphatase and urease activities in all particle-size fractions (2000-250, 250-53, 53-2 and < 2 μm) compared with the CTRR treatment. Both β-glucosidase and acid phosphatase showed a general decrease from coarse- to fine-sized fractions, and resembled the distribution of SOC and TN concentrations in particle-size fractions. Conversely, urease activity was greater in sand and clay fractions, which was decoupled from SOC and TN distributions. Our results indicate that biological indicators of soil health were more sensitive than C and N stocks to cumulative long-term changes in tillage and residue management.
Phosphatase enzymes play a key role cycling phosphorus from organic to plant-available pools, particularly in tropical soils where inorganic phosphorus is often limited. However, most studies of ...phosphatase activity have focused only on surface soils, despite the large quantities of carbon and nutrients stored in tropical subsoils. The goal of this study was to determine how acid phosphatase kinetic parameters change with depth across two parent materials (represented by Oxisols and an Inceptisols) and two distinct forests (lower and upper montane) at the Luquillo Critical Zone Observatory in northeast Puerto Rico. We collected samples from five soil pits at each of four soil × forest types, and measured apparent phosphatase kinetic parameters (AppVmax and AppKm) and soil nutrients at 0, 20, 50, 80, 110 and 140 cm depths. Across all sites, AppVmax declined 97% and AppKm declined 85% from the surface to 140 cm depth. The ratio of AppVmax to AppKm (i.e., Ka) did not change through the first meter of soil profiles but was significantly reduced by 50% 140 cm. Total carbon, nitrogen and extractable phosphorus all declined exponentially with depth. Carbon concentrations and AppVmax were both significantly greater in Oxisols compared with Inceptisols, and in the higher elevation montane forest compared to the lower elevation forest. The scaling relationship we observe between AppVmax and AppKm is common for environmental systems, although the degree of correlation in our study (R2 = 0.48) is unusually high, suggesting these parameters are both driven by changes in energy and nutrient availability along depth profiles. However, the consistency of Ka with depth indicates that overall catalytic capacity of phosphatase is maintained across a range of substrate concentrations. The larger variability in AppVmax compared with AppKm suggests microorganisms exert more control over phosphatase production than substrate availability. Our findings indicate that subsoil microbial communities are not metabolically dormant, but rather contribute to P-cycling at rates comparable to their surface counterparts. Further research on ecology of microorganisms in resource-limited tropical subsoils is warranted to better understand microbial contributions to biogeochemical cycles throughout tropical soil profiles.
•Measured changes in phosphatase kinetics with depth in two tropical forest soils.•AppVmax, AppKm, carbon and phosphorus concentrations declined with depth.•Catalytic capacity of phosphatase (Ka) is constant within the first meter of soils.•Vmax is more plastic than Km in response to changing soil resource availability.
Soil organic matter (SOM) turnover increasingly is conceptualized as a tension between accessibility to microorganisms and protection from decomposition via physical and chemical association with ...minerals in emerging soil biogeochemical theory. Yet, these components are missing from the original mathematical models of belowground carbon dynamics and remain underrepresented in more recent compartmental models that separate SOM into discrete pools with differing turnover times. Thus, a gap currently exists between the emergent understanding of SOM dynamics and our ability to improve terrestrial biogeochemical projections that rely on the existing models. In this opinion paper, we portray the SOM paradigm as a triangle composed of three nodes: conceptual theory, analytical measurement, and numerical models. In successful approaches, we contend that the nodes are connected—models capture the essential features of dominant theories while measurement tools generate data adequate to parameterize and evaluate the models—and balanced— models can inspire new theories via emergent behaviors, pushing empiricists to devise new measurements. Many exciting advances recently pushed the boundaries on one or more nodes. However, newly integrated triangles have yet to coalesce. We conclude that our ability to incorporate mechanisms of microbial decomposition and physicochemical protection into predictions of SOM change is limited by current disconnections and imbalances among theory, measurement, and modeling. Opportunities to reintegrate the three components of the SOM paradigm exist by carefully considering their linkages and feedbacks at specific scales of observation.
The soil C saturation concept suggests a limit to whole soil organic carbon (SOC) accumulation determined by inherent physicochemical characteristics of four soil C pools: unprotected, physically ...protected, chemically protected, and biochemically protected. Previous attempts to quantify soil C sequestration capacity have focused primarily on silt and clay protection and largely ignored the effects of soil structural protection and biochemical protection. We assessed two contrasting models of SOC accumulation, one with no saturation limit (i.e., linear first-order model) and one with an explicit soil C saturation limit (i.e., C saturation model). We isolated soil fractions corresponding to the C pools (i.e., free particulate organic matter POM, microaggregate-associated C, silt- and clay-associated C, and nonhydrolyzable C) from eight long-term agroecosystem experiments across the United States and Canada. Due to the composite nature of the physically protected C pool, we fractioned it into mineral- vs. POM-associated C. Within each site, the number of fractions fitting the C saturation model was directly related to maximum SOC content, suggesting that a broad range in SOC content is necessary to evaluate fraction C saturation. The two sites with the greatest SOC range showed C saturation behavior in the chemically, biochemically, and some mineral-associated fractions of the physically protected pool. The unprotected pool and the aggregate-protected POM showed linear, nonsaturating behavior. Evidence of C saturation of chemically and biochemically protected SOC pools was observed at sites far from their theoretical C saturation level, while saturation of aggregate-protected fractions occurred in soils closer to their C saturation level.
This study assessed the applicability of artificial neural networks (ANNs) as a tool to identify compounds contributing to compositional differences in coal-contaminated soils. An artificial neural ...network model was constructed from laser desorption ionization ultrahigh-resolution mass spectra obtained from coal contaminated soils. A good correlation (R2 = 1.00 for model and R2 = 0.99 for test) was observed between the measured and predicted values, thus validating the constructed model. To identify chemicals contributing to the coal contents of the soils, the weight values of the constructed model were evaluated. Condensed hydrocarbon and low oxygen containing compounds were found to have larger weight values and hence they were the main contributors to the coal contents of soils. In contrast, compounds identified as lignin did not contribute to the coal contents of soils. These findings were consistent with the conventional knowledge on coal and results from the conventional partial least square method. Therefore, we concluded that the weight interpretation following ANN analysis presented herein can be used to identify compounds that contribute to the compositional differences of natural organic matter (NOM) samples.
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•Molecular differences in coal-contaminated soils studied with ANN and FT-ICR MS.•First study to use ANN analysis to study NOM spectra obtained from FT-ICR MS.•Weight analysis applied identify compounds in FT-ICR MS spectra.•Compounds identified as lignin did not contribute to the coal contents of soil.•Condensed hydrocarbon and low oxygenated compounds contributed to coal in soil.
Alluvial riparian soils act as a filtration system, improving the environmental quality of downstream soils and waters. In areas affected by coal mining, alluvial soils also serve as a modern “sink” ...of fossil carbon (C). To date, little research has been done on ecosystem services provided by alluvial landscapes (i.e., river islands and tributary deltas) in the retention of coal in coal‐mining regions. The objective of this study was to distinguish between and quantify geogenic and neogenetic C in alluvial soils of the North Branch of the Susquehanna River (NBSR). To investigate this, we compared five thermal analysis methods to quantify geogenic (coal) C in soils. Our results indicate that multivariate curve resolution of ramped thermal combustion data provided the most accurate estimate of coal content in soils. Our analysis found that NBSR alluvial soils have accumulated ∼375 Gg of anthropogenic, geogenic C (upper 1 m). In these soils, an average of ∼11% of soil mass is attributable to coal, yet ∼73% of the total soil C is attributable to geogenic C. These soil organic C stocks are substantially greater than locally mapped riparian soils unaffected by coal mining and are greater than regional organic soils (Histosols). Quantification of microbial decomposition of coal in alluvial soils and vulnerability to extreme flood events (potential remobilization) requires further investigation and will be important in determining the fate of this C sink.
Core Ideas
Alluvial soils downstream of mining areas are sinks for waste coal.
Coal contributed 11% of soil mass and 73% of total C in sampled soils.
North Branch of Susquehanna River alluvial soils contain 815 Mg geogenic C ha−1.
These soils have also accumulated 266 Mg neogenetic soil C ha−1.
Distinguishing and quantifying these two pools is essential to predicting fate of C sink.
•Forest biomass in Delaware River Basin was a carbon sink over the past decade.•Change of biomass C stocks did not correlate with climatic or topographic factors.•Demographic changes in tree size and ...species were main drivers of biomass C change.
Quantifying forest biomass carbon (C) stock change is important for understanding forest dynamics and their feedbacks with climate change. Forests in the northeastern U.S. have been a net carbon sink in recent decades, but C accumulation in some northern hardwood forests has been halted due to the impact of emerging stresses such as invasive pests, land use change and climate change. The Delaware River Basin (DRB), sited in the southern edge of the northern hardwood forest, features diverse forest types and land-use histories. In 2001–2003, the DRB Monitoring and Research Initiative established 61 forest plots in three research sites, using Forest Service inventory protocols and enhanced measurements. These plots were revisited and re-measured in 2012–2014. By comparing forest biomass C stocks in the two measurements, our results suggest that the biomass C stock of the DRB forest increased, and was thus a carbon sink over the past decade. The net biomass C stock change in the Neversink area in the north of the DRB was 1.94MgCha−1yr−1, smaller than the biomass C change in the French Creek area (2.52MgCha−1yr−1, southern DRB), and Delaware Water Gap Area (2.68MgCha−1yr−1, central DRB). An increase of dead biomass C accounted for 20% of the total biomass C change. The change of biomass C stocks did not correlate with any climatic or topographic factors, but decreased with increasing stand age, and with tree mortality rate. Mortality rates were highest in the smallest size class. In most of the major tree species, stem density decreased, but the loss of biomass from mortality was offset by recruitment and growth. The demographic changes differ dramatically among species. The living biomass of chestnut oak, white oak and black oak decreased because of the large mortality rate, while white pine, American beech and sweet birch increased in both biomass and stem density. Our results suggest that forests in the DRB could continue to be a carbon sink in the coming decades, because they are likely at a middle successional stage. The linkage between demography of individual trees species and biomass C change underscores the effects of species-specific disturbances such as non-native insects and pathogens on forest dynamics, and highlights the need for forest managers to anticipate these effects in their management plans.
Previous research on the protection of soil organic C from decomposition suggests that soil texture affects soil C stocks. However, different pools of soil organic matter (SOM) might be differently ...related to soil texture. Our objective was to examine how soil texture differentially alters the distribution of organic C within physically and chemically defined pools of unprotected and protected SOM. We collected samples from two soil texture gradients where other variables influencing soil organic C content were held constant. One texture gradient (16-60% clay) was located near Stewart Valley, Saskatchewan, Canada and the other (25-50% clay) near Cygnet, OH. Soils were physically fractionated into coarse- and fine-particulate organic matter (POM), silt- and clay-sized particles within microaggregates, and easily dispersed silt- and clay-sized particles outside of microaggregates. Whole-soil organic C concentration was positively related to silt plus clay content at both sites. We found no relationship between soil texture and unprotected C (coarse- and fine-POM C). Biochemically protected C (nonhydrolyzable C) increased with increasing clay content in whole-soil samples, but the proportion of nonhydrolyzable C within silt- and clay-sized fractions was unchanged. As the amount of silt or clay increased, the amount of C stabilized within easily dispersed and microaggregate-associated silt or clay fractions decreased. Our results suggest that for a given level of C inputs, the relationship between mineral surface area and soil organic matter varies with soil texture for physically and biochemically protected C fractions. Because soil texture acts directly and indirectly on various protection mechanisms, it may not be a universal predictor of whole-soil C content.
Although current assessments of agricultural management practices on soil organic C (SOC) dynamics are usually conducted without any explicit consideration of limits to soil C storage, it has been ...hypothesized that the SOC pool has an upper, or saturation limit with respect to C input levels at steady state. Agricultural management practices that increase C input levels over time produce a new
equilibrium soil C content. However, multiple C input level treatments that produce no increase in SOC stocks at equilibrium show that soils have become
saturated with respect to C inputs. SOC storage of added C input is a function of how far a soil is from saturation level (saturation deficit) as well as C input level. We tested experimentally if C saturation deficit and varying C input levels influenced soil C stabilization of added
13C in soils varying in SOC content and physiochemical characteristics. We incubated for 2.5 years soil samples from seven agricultural sites that were closer to (i.e., A-horizon) or further from (i.e., C-horizon) their C saturation limit. At the initiation of the incubations, samples received low or high C input levels of
13C-labeled wheat straw. We also tested the effect of Ca addition and residue quality on a subset of these soils. We hypothesized that the proportion of C stabilized would be greater in samples with larger C saturation deficits (i.e., the C- versus A-horizon samples) and that the relative stabilization efficiency (i.e., ΔSOC/ΔC input) would decrease as C input level increased. We found that C saturation deficit influenced the stabilization of added residue at six out of the seven sites and C addition level affected the stabilization of added residue in four sites, corroborating both hypotheses. Increasing Ca availability or decreasing residue quality had no effect on the stabilization of added residue. The amount of new C stabilized was significantly related to C saturation deficit, supporting the hypothesis that C saturation influenced C stabilization at all our sites. Our results suggest that soils with low C contents and degraded lands may have the greatest potential and efficiency to store added C because they are further from their saturation level.
A large fraction of soil organic matter (OM) resists decomposition over decades to centuries as indicated by long radiocarbon residence times, but the mechanisms responsible for the long-term ...(multidecadal) persistence are debated. The current lack of mechanistic understanding limits our ability to accurately predict soil OM stock evolution under climate and land-use changes. Using a unique set of historic soil samples from five long-term (27–79 years) bare fallow experiments, we demonstrate that despite wide pedo-climatic diversity, persistent OM shows specific energetic signatures, but no uniform chemical composition. From an energetic point of view, thermal analyses revealed that combustion of persistent OM occurred at higher temperature and provided less energy than combustion of more labile OM. In terms of chemical composition, persistent OM was H-depleted compared to OM present at the start of bare fallow, but spectroscopic analyses of OM functional groups did not reflect a consistent chemical composition of OM across sites, nor substantial modifications with bare fallow duration. The low energy content of persistent OM may be attributed to a combination of reduced content of energetic C–H bonds or stronger interactions between OM and the mineral matrix. Soil microorganisms thus appear to preferentially mineralize high-energy OM, leaving behind material with low energy content. This study provides the first direct link between long-term persistence of OM in soil and the energetic barriers experienced by the decomposer community.
