Recovery Of Soil Organic Matter Research Articles

Litterfall production, decomposition and litter nutrient contents in a mined area revegetated with different forest species

Afforestation of sites disturbed after bauxite mining is the favorite technique to restore all ecosystem functions. The nature of the tree species used for revegetation of post-mining land can accelerate the recovery of soil organic matter and nutrient cycles. This study aimed to determine the litterfall production, decomposition rate and nutrient content from three types of forest cover ( Eucalyptus , Anadenanthera peregrina , mixed plantation of 16 native species) planted in a bauxite mining area in recovery. Litterfall [...]

Extraction methods for untargeted metabolomics influence enzymatic activity in diverse soils

Soil organic matter (SOM) is the largest carbon pool in terrestrial ecosystems and underpins the health and productivity of soil. Accurate characterization of its chemical composition will improve our understanding of biotic and abiotic processes regulating its stabilization. Our purpose in this study was to estimate the loss of SOM by microbial and exoenzymatic activity that might occur when soil is extracted for analysis of representative low molecular weight mass features using untargeted metabolomics. Two mined clays (kaolinite, montmorillonite) and three diverse soils (varying in texture, specific surface area and cation exchange capacity) were used to assess the extraction efficiency and loss of three enzymatic activity indicators (2,6-dichloroindophenol sodium salt hydrate [DCIP], 4-methylumbelliferyl phosphate [MUBph] and 3,4-dihydroxy-L-phenylalanine [LDOPA]) during extraction with two different solvents (water and methanol). Losses of the indicators were attributed to extraction method (ultrasonication, shaking, or shaking following chloroform fumigation), physical properties associated with the soil/clay type, and microbial activity. Soil/clay type strongly influenced indicator recovery and hence, SOM recovery. Choice of extraction method strongly influenced the composition and recovery of representative SOM mass features, while the choice of solvent determined whether the soil type or extraction method had a greater influence of compositional differences in the SOM mass features extracted. Extraction following chloroform fumigation had the greatest loss of the indicators, due to enzymatic activity and/or adsorption onto the soil matrix. Minimal variation in composition and loss of SOM mass features occurred during extraction by shaking for the soils tested; we therefore recommend it as the method of choice for untargeted SOM extraction studies.

Monitoring soil resilience via the dynamic changes of selected physicochemical properties of soil in a tropical rehabilitated forest

Recovery of soil organic matter and mineral nutrient cycling are critical to the success of rehabilitation process of replanted forest. We investigated the dynamic changes in soil of selected physicochemical properties including organic carbon (C), nitrogen (N), phosphorus (P) and sulphur (S) of replanted forest rehabilitation that had been previously disturbed by anthropogenic activities. Soil samples were collected from the rehabilitated forest of three ages stands (3-, 12- and 23-years old) while samples from adjacent secondary forest was collected for comparison. Altogether, 36 samples were taken randomly with a soil auger at depths of 0-20 cm from a plot of 20 × 20 m2 at each site. Soils were analyzed for pH, bulk density, C, N, P and S. The data obtained were statistically analyzed using ANOVA with Tukey’s test performed by SAS 9.2 at P ≤ 0.05. Results showed that total C, N, P, C/N ratio and C/S ratio increased with age of rehabilitated forest except for pH, S and C/P ratio. Forest rehabilitation by planting indigenous tree species has shown a potential of recovery, but further investigation into the process control of the dynamic changes of soil physicochemical properties, particularly in the event of further ecosystem disturbance is needed.

Effects of technosols based on organic amendments addition for the recovery of the functionality of degraded quarry soils under semiarid Mediterranean climate: A field study

This study aims to evaluate the effects of technosols made with different organic amendments to restore degraded soils in a semiarid limestone quarry. The effects on soil quality, functionality and organic matter dynamics of the technosols amended with waste of gardening, greenhouse horticultural, stabilized sewage sludge and two mixtures of sludge with both vegetable composts were assessed. Several physical and chemical properties, humus fractions, soil respiration and molecular composition was performed after 6 and 18 months. Un-amended soils, and nearby natural undegraded soils served as reference. Amended technosols increased water retention capacity, electrical conductivity, total organic carbon and nitrogen, respect to not amended and natural soils. Humus fraction composition was not altered over time. Un-amended soils, very poor in organic matter, did not show any pyrolyzable compounds or labile soil organic matter by thermogravimetry. In contrast, the pyrochromatograms of natural soils showed lignocellulosic materials, polypeptides and a noticeable presence of alkylic compounds. In technosols with both types vegetable compost, the organic matter structure was more complex, showing compounds from lignin-derived and long-chain alkyl, polysaccharides, chlorophyll isoprenoids and nitrogen. In sludge technosol, a set of sterols was outstanding. The mixtures showed a molecular fingerprint of materials derived from the decomposition of the organic amendments that formed them. These signs of the contribution of different organic matter forms derived from the amendments were also reported by the series exothermic peaks found in the calorimetry. This short-term study indicates a clear effect of the amendments on the recovery of soil organic matter and presumably of its functionality. After the amendments application, microbial activity and soil respiration rates increased rapidly but ceased 18 months later. The molecular composition of the organic matter of the soils amended with plant compost was very similar to that of natural, non-degraded soils in nearby areas.

Seasonal evolution of soil organic matter, glomalin and enzymes and potential for C storage after land abandonment and renaturalization processes in soils of NE Spain

Rates of recovery of soil organic matter (SOM) in abandoned fields of Mediterranean Spain are affected by spontaneous succession in the vegetation, the occurrence of periodic wildfires and lack of forest management. Soil organic matter has a relevant role together with the renaturalization process of shallow soil under vines (V) and olive groves (OL), cork oak (S) and pine (PI) trees, pasture (PR), Cistus (MC) and Erica (MB) scrub in abandoned areas of NE Spain that are sensitive to fire. Physical, chemical, biological and biochemical properties of the soil determined season by season warned against its fragility. Pyrolysis-GC products showed in general moderate to strong positive and negative correlation with soil organic carbon (OC), easily extractable glomalin (EE-GRSP) and total glomalin (GRSP), β-glucosidase and protease, which strengthens the hypothesis that both less and more recalcitrant SOM fractions might be inferred and so there is a scale of potential carbon storage in the environments studied.Recently abandoned V and OL soil had small OC contents (mean 2.51±0.25 and 15.26±1.39gkg−1) and GRSP (mean 1.04±0.22 and 2.84±0.53gkg−1) and large contents of aromatic compounds. By contrast, PR and MB soil had large OC (mean 36.24±0.75 and 37.66±2.38gkg−1) and GRSP (mean 4.85±0.87 and 3.88±0.76gkg−1) content and smaller contents of aromatic compounds. This suggested a renewal of active OC in older soil. In fact, EE-GRSP and GRSP of more organic soil were positively correlated with the mineralization indices N/O (Furfural/Pyrrole) and O/Y (Pyrrole/Phenol) from active organic compounds (P<0.05 and P<0.01 respectively), and negatively correlated (P<0.05) with the humification index B/E3 (Benzene/Toluene) from recalcitrant organic compounds. Principal component analysis (PCA) identified those environments where soil renaturalization is better. Our findings might indicate the potential for carbon storage in different abandoned shallow soil environments under renaturalization in Mediterranean climate conditions.

Post-fire recovery of soil organic matter in a Cambisol from typical Mediterranean forest in Southwestern Spain

Wildfire is a recurrent phenomenon in Mediterranean ecosystems and contributes to soil degradation and desertification, which are partially caused by alterations to soil organic matter (SOM). The SOM composition from a Cambisol under a Mediterranean forest affected by a wildfire is studied in detail in order to assess soil health status and better understand of soil recovery after the fire event. The soil was sampled one month and twenty-five months after the wildfire. A nearby unburnt site was taken as control soil. Soil rehabilitation actions involving heavy machinery to remove burnt vegetation were conducted sixteen months after the wildfire. Immediately after fire the SOM increased in topsoil due to inputs from charred vegetation, whereas a decrease was observed in the underlying soil layer. Twenty-five months after fire soil-pH increased in fire-affected topsoil due to the presence of ashes, a decrease in SOM content was recorded for the burnt topsoil and similar trend was observed for the water holding capacity. The pyro-chromatograms of burned soils revealed the formation of additional aromatic compounds. The thermal cracking of long-chain n-alkanes was also detected. Solid-state 13C NMR spectroscopy supported the increase of aromatic compounds in the fire-affected topsoil due to the accumulation of charcoal, whereas the deeper soil sections were not affected by the fire. Two years later, soil parameters for the unburnt and burnt sites showed comparable values. The reduction of the relative intensity in the aromatic C region of the NMR spectra indicated a decrease in the charcoal content of the topsoil. Due to the negligible slope in the sampling site, the loss of charcoal was explained by the post-fire restoration activity, degradation, leaching of pyrogenic SOM into deeper soil horizons or wind erosion. Our results support that in the Mediterranean region, fire-induced alteration of the SOM is not lasting in the long-term.

Corrigendum to: The long-term impact of low-intensity surface fires on litter decomposition and enzyme activities in boreal coniferous forests

In boreal forest ecosystems fire, fungi and bacteria, and their interactions, have a pronounced effect on soil carbon dynamics. In this study we measured enzymatic activities, litter decomposition rates, carbon stocks and fungal and microbial biomasses in a boreal subarctic coniferous forest on a four age classes of non-stand replacing fire chronosequence (2, 42, 60 and 152 years after the fire). The results show that microbial activity recovered slowly after fire and the decomposition of new litter was affected by the disturbance. The percent mass loss of Scots pine litter increased with time from the last fire. Slow litter decomposition during the first post-fire years accelerates soil organic matter accumulation that is essential for the recovery of soil biological activities. Fire reduced the enzymatic activity across all the enzyme types measured. Carbon-degrading, chitin-degrading and phosphorus-dissolving enzymes showed different responses with the time elapsed since the fire disturbance. Microbial and enzymatic activity took decades before recovering to the levels observed in old forest stands. Our study demonstrates that slower post-fire litter decomposition has a pronounced impact on the recovery of soil organic matter following forest fires in northern boreal coniferous forests.

Development of Alternative Methods for Determining Soil Organic Matter

Soil organic matter (SOM) is important to fertility, since it performs several functions such as cycling, water and nutrient retention and soil aggregation, in addition to being an energy requirement for biological activity. This study proposes new trends to the Embrapa, Walkley-Black, and Mebius methods that allowed the determination of SOM by spectrophotometry, increasing functionality. The mass of 500 mg was reduced to 200 mg, generating a mean of 60 % saving of reagents and a decrease of 91 % in the volume of residue generated for the three methods without compromising accuracy and precision. We were able to optimize conditions for the Mebius method and establish the digestion time of maximum recovery of SOM by factorial design and response surface. The methods were validated by the estimate of figures of merits. Between the methods investigated, the optimized Mebius method was best suited for determining SOM, showing near 100 % recovery.

The long-term impact of low-intensity surface fires on litter decomposition and enzyme activities in boreal coniferous forests

In boreal forest ecosystems fire, fungi and bacteria, and their interactions, have a pronounced effect on soil carbon dynamics. In this study we measured enzymatic activities, litter decomposition rates, carbon stocks and fungal and microbial biomasses in a boreal subarctic coniferous forest on a four age classes of non-stand replacing fire chronosequence (2, 42, 60 and 152 years after the fire). The results show that microbial activity recovered slowly after fire and the decomposition of new litter was affected by the disturbance. The percent mass loss of Scots pine litter increased with time from the last fire. Slow litter decomposition during the first post-fire years accelerates soil organic matter accumulation that is essential for the recovery of soil biological activities. Fire reduced the enzymatic activity across all the enzyme types measured. Carbon-degrading, chitin-degrading and phosphorus-dissolving enzymes showed different responses with the time elapsed since the fire disturbance. Microbial and enzymatic activity took decades before recovering to the levels observed in old forest stands. Our study demonstrates that slower post-fire litter decomposition has a pronounced impact on the recovery of soil organic matter following forest fires in northern boreal coniferous forests.

Recovery of arctic tundra from thermal erosion disturbance is constrained by nutrient accumulation: a modeling analysis.

Abstract. We calibrated the Multiple Element Limitation (MEL) model to Alaskan arctic tundra to simulate recovery of thermal erosion features (TEFs) caused by permafrost thaw and mass wasting. TEFs could significantly alter regional carbon (C) and nutrient budgets because permafrost soils contain large stocks of soil organic matter (SOM) and TEFs are expected to become more frequent as the climate warms. We simulated recovery following TEF stabilization and did not address initial, short-term losses of C and nutrients during TEF formation. To capture the variability among and within TEFs, we modeled a range of post-stabilization conditions by varying the initial size of SOM stocks and nutrient supply rates. Simulations indicate that nitrogen (N) losses after the TEF stabilizes are small, but phosphorus (P) losses continue. Vegetation biomass recovered 90% of its undisturbed C, N, and P stocks in 100 years using nutrients mineralized from SOM. Because of low litter inputs but continued decomposition, younger SOM continued to be lost for 10 years after the TEF began to recover, but recovered to about 84% of its undisturbed amount in 100 years. The older recalcitrant SOM in mineral soil continued to be lost throughout the 100-year simulation. Simulations suggest that biomass recovery depended on the amount of SOM remaining after disturbance. Recovery was initially limited by the photosynthetic capacity of vegetation but became co-limited by N and P once a plant canopy developed. Biomass and SOM recovery was enhanced by increasing nutrient supplies, but the magnitude, source, and controls on these supplies are poorly understood. Faster mineralization of nutrients from SOM (e.g., by warming) enhanced vegetation recovery but delayed recovery of SOM. Taken together, these results suggest that although vegetation and surface SOM on TEFs recovered quickly (25 and 100 years, respectively), the recovery of deep, mineral soil SOM took centuries and represented a major ecosystem C loss.

Particulate organic carbon at reclaimed and unreclaimed post-mining soils and its microbial community composition

Recovery of soil organic matter and associated microbial biomass is a fundamental precondition for successful restoration of post-mining soils. The aim is to compare the dynamics of soil C fractions and of microbial communities associated with these fractions in two chronosequences of post-mining sites with different plant communities. Soil carbon, pH, bulk density and the light fraction of particulate organic carbon (POC), free or bound in soil aggregates, were studied along two chronosequences, both covering successional ages from 10 to 50years. One chronosequence had been reclaimed by planting of alder, while the other had been vegetated by natural regrowth (Salix caprea, Populus tremula and Betula pendula). In intermediate and late successional stages, microbial community in bulk soil and POC fractions were studied using phospholipid fatty acid analysis. Soil C content increased and pH decreased with plot age, these trends being more pronounced at reclaimed sites. The light and bound POC fractions increased with age, higher values and a larger increase being found at reclaimed sites. In both chronosequences, the light fraction was an order of magnitude higher than the bound fraction. C content in both fractions increased with successional age, with higher C content at reclaimed sites. Microbial communities were more affected by the POC fraction than plot age. The bulk soil of reclaimed sites was more similar to bound POC, while the bulk soil of unreclaimed soils was similar to the light POC fraction. Observed differences correspond with a higher level of bioturbation at the reclaimed sites, which promotes faster accumulation of bound POC and drives bulk soil microbial communities closer to those of bound POC.

Natural recovery of soil organic matter in 30–90‐year‐old abandoned oil and gas wells in sagebrush steppe

We addressed the rarely studied issue of how different soil organic matter pools respond to disturbances from historical oil and gas well development in semi‐arid Intermountain sagebrush steppe. We selected twenty‐nine study well sites in south‐central Wyoming that were plugged and abandoned 33–90 years ago. We designed our study to understand the long term impact of oil and gas development for soil organic matter pools on non‐reclaimed sites, and evaluate the importance of this disturbance type relative to other major influences on soil organic matter and the fine‐scale, shrub‐induced heterogeneity of soil organic matter. We compared total, labile, and recalcitrant pools of soil C and N in disturbed sites to adjacent, un‐disturbed sites. We found that that natural site‐specific conditions such as soil texture and fine‐scale heterogeneity associated with shrubs are the most important controls over soil C and N, particulate organic matter C and N, and potential C and N mineralization, and that these older well‐pad disturbances did not have a significant effect on any soil organic matter pools. Fine‐scale, shrub‐induced heterogeneity was higher for those pools that have a fast turnover rate than those with a slow turnover rate. Moreover, fine‐scale heterogeneity of total soil C was higher for well sites that were located in loamy sand soils compared to well sites on sandy soils. In addition, heterogeneity of total soil C recovered through time on the finer textured soils. Overall our study suggests that soil organic matter pools were not affected by old oil well development, and that recovery of shrub cover is a key component of soil organic matter recovery in these semi‐arid shrublands. A comparison of our work to other work on recent and reclaimed sites suggests that some reclamation procedures may decrease soil organic matter more than the absence of reclamation.

Evolution of soil organic matter after prescribed fire: A 20-year chronosequence

Vegetation fires influence the properties and turnover of soil organic matter in numerous ecosystems. Thermal alteration tends to increase the stability of soil organic matter. Fire induces the destruction of above‐ground biomass, which is associated with a large production of new partially-charred litter. Plant regeneration is directly affected by the fire itself, and by the ensuing modifications to the C, N and nutrient cycles associated with soil organic matter transformations. The objectives of the present study were to determine 1) changes in soil organic matter composition induced by fire, and 2) the dynamics of soil organic matter recovery in the first two decades following a fire event.. In a Florida scrub oak ecosystem, a chronosequence of soils protected from vegetation fire for 1 to 20years was studied. The bulk organic matter and oxidation resistant elemental carbon were quantified in depths 0–5cm, 5–15cm, and 15–25cm. The soil organic matter was characterized by solid-state 13C NMR spectroscopy and Curie point pyrolysis.The combination of these techniques allowed us to identify three steps in soil organic matter evolution: first the un-charred litter brought by the fire degraded between 1 and 4years after fire; second the contribution of aryl carbon, and most probably the pyrogenic carbon, significantly decreased between 4 and 11years after fire; Thirdly, after 11years, the soil organic matter quality appeared driven again by the fresh litter input from regenerating plant ecosystem. In this study, the soil organic matter did not appear strongly modified by the fire. The pyrogenic carbon did not dominate the soil organic matter composition and underwent significant degradation at the decadal timescale. Our results also highlight a potential underestimated effect of dead root input to soil organic matter after the fire.

Recovery of soil organic matter, organic matter turnover and nitrogen cycling in a post-mining forest rehabilitation chronosequence

Recovery of soil organic matter, organic matter turnover and mineral nutrient cycling is critical to the success of rehabilitation schemes following major ecosystem disturbance. We investigated successional changes in soil nutrient contents, microbial biomass and activity, C utilisation efficiency and N cycling dynamics in a chronosequence of seven ages (between 0 and 26 years old) of jarrah ( Eucalyptus marginata) forest rehabilitation that had been previously mined for bauxite. Recovery was assessed by comparison of rehabilitation soils to non-mined jarrah forest references sites. Mining operations resulted in significant losses of soil total C and N, microbial biomass C and microbial quotients. Organic matter quantity recovered within the rehabilitation chronosequence soils to a level comparable to that of non-mined forest soil. Recovery of soil N was faster than soil C and recovery of microbial and soluble organic C and N fractions was faster than total soil C and N. The recovery of soil organic matter and changes to soil pH displayed distinct spatial heterogeneity due to the surface micro-topography (mounds and furrows) created by contour ripping of rehabilitation sites. Decreases in the metabolic quotient with rehabilitation age conformed to conceptual models of ecosystem energetics during succession but may have been more indicative of decreasing C availability than increased metabolic efficiency. Net ammonification and nitrification rates suggested that the low organic C environment in mound soils may favour autotrophic nitrifier populations, but the production of nitrate (NO 3 −) was limited by the low gross N ammonification rates (≤1 μg N g −1 d −1). Gross N transformation rates in furrow soils suggested that the capacity to immobilise N was closely coupled to the capacity to mineralise N, suggesting NO 3 − accumulation in situ is unlikely. The C:N ratio of the older rehabilitation soils was significantly lower than that of the non-mined forest soils. However, variation in ammonification rates was best explained by C and N quantity rather than C:N ratios of whole soil or soluble organic matter fractions. We conclude that the rehabilitated ecosystems are developing a conservative N cycle as displayed by non-mined jarrah forests. However, further investigation into the control of nitrification dynamics, particularly in the event of further ecosystem disturbance, is warranted.

Organic Amendment and Rotation Crop Effects on the Recovery of Soil Organic Matter and Aggregation in Potato Cropping Systems

Soil structural degradation is common in intensively cultivated ecosystems due to the depletion of soil organic matter (SOM). We investigated the mechanisms by which different frequencies of organic amendment application and rotation crops restore C, N, and aggregation in gravelly loam soils used for potato production. A single amendment application [FIRST; 22 Mg ha−1 compost and 45 Mg ha−1 beef cattle (Bos taurus L.) manure] did not affect total C in 1996 and increased it by 28% in 1997 relative to unamended plots (NONE); light fraction (LF) C accounted for 56% of this increase. Plots in which amendment was suspended for 1 yr (SASP) following 4 or 5 yr of annual application had more total C in 1996 (28%) and 1997 (46%) relative to NONE. A green manure crop consisting of oat (Avena sativa ‘Porter’), pea (Pisum sativum ‘Trapper’), and hairy vetch (Vicia villosa Roth) grown in 2‐yr rotation with potatoes (Solanum tuberosum L.) increased soil C in 1997 (25.9 vs. 23.9 g kg−1), LF properties in 1996 and 1997, and water soluble carbohydrates (WSC) on several sample dates relative to an oat rotation crop. Large aggregate (2–6.5 mm) stability in 1996 and 1997 and medium aggregate (1–2 mm) stability in 1997 were increased by FIRST relative to NONE. Total soil C was more strongly related to medium (r = 0.65 in 1997) and large (r = 0.51 in 1997) aggregate stability than LF or water soluble carbohydrate fractions. Compost and manure influences occurred rapidly and were persistent, demonstrating that annual applications are not necessary to reverse soil degradation.

Soil Organic Matter Recovery on Conservation Reserve Program Fields in Southeastern Wyoming

AbstractSoil C and N changes following cessation of cultivation in semiarid soils is not well understood. We hypothesized that returning cultivated fields in southeastern Wyoming to perennial grasses through the Conservation Reserve Program (CRP) would (i) increase labile pools of soil organic matter (SOM), and (ii) increase small‐scale heterogeneity of SOM. Carbon and N in labile and passive pools of SOM were measured in CRP fields seeded with perennial grasses intermediate wheatgrass (Elytrigia intermedia [Host] Nevski ssp. intermedia), pubescent wheatgrass (Elytrigia intermedia [Schur.] A. Löve ssp. barbulata) and smooth brome (Bromus inermis Leysser), and in winter wheat (Triticum aestivum L.)‐fallow fields. Mineralizable C increased from 0.37 g m−2 d−1 in wheat‐fallow fields to 0.99 g m−2 d−1 in CRP fields; mineralizable N and coarse particulate C were consistently but not significantly higher in CRP fields. Fine particulate and total soil C and N were not significantly different between CRP and wheat‐fallow. Within CRP fields, mineralizable C was significantly higher under grasses than in interspaces (1.96 vs. 0.73 g m−2 d−1, respectively), and mineralizable N and coarse particulate C and N were consistently but not significantly higher under grasses than in interspaces. Soil C and N have increased only slightly after 6 yr of CRP management, and future changes in land use management on these CRP fields, including grazing and cropping, may accrue some small benefits associated with improved soil fertility status.

Legume, Grass, and Conservation Reserve Program Effects on Soil Organic Matter Recovery

Active pools of soil organic matter (SOM) can recover to native levels on formerly cultivated fields that are abandoned for ≈50 yr, but the short-term (<10 yr) recovery dynamics of SOM and nutrient supply have not been widely investigated. In several fields on a farm in southeastern Wyoming that had been involved in the Conservation Reserve Program (CRP, a federal program that pays landowners to convert cultivated land into revegetated grasslands), we compared C and N in several SOM pools (coarse particulate organic matter [POM, between 500 μm and 2 mm], fine POM [53–500 μm], and total SOM), and we compared potential C and N mineralization in active pools responsible for nutrient supply. The two CRP treatments, planted 6 yr prior to this study, were an ≈80% legume:20% grass mixture (HL CRP) and a 20% legume:80% grass mixture (LL CRP). To quantify SOM accumulations directly due to increased plant inputs within CRP fields, we also compared SOM pools under legumes and grasses relative to plant interspaces, where we expected plant inputs to be minimal. The net impacts of increased plant inputs and the cessation of tillage generally increased pools of mineralizable and coarse-POM C and N by factors of two to four relative to wheat–fallow fields (alternate years in winter wheat and in fallow), but had negligible effects on total SOM. Recovery of microsite (≈10-cm scale) soil heterogeneity, an important structural attribute of native arid and semiarid ecosystems, was accelerated under legumes, which produced more labile tissue than grasses. Soils under legumes contained larger pools of coarse-POM C and N and exhibited higher net N mineralization rates than soil under grasses or in plant interspaces. Grasses grown in HL CRP soils, which had the highest rates of potential net N mineralization, produced more labile tissue than the same grasses grown in the more nutrient-depleted LL CRP fields, suggesting that plant/soil feedbacks were important. Therefore, recovery of labile soil and plant N was enhanced when the proportion of legumes was high, and this may lead to improved grain or animal N nutrition if these CRP fields are subsequently cropped or grazed.

Soil Organic Matter Recovery in Semiarid Grasslands: Implications for the Conservation Reserve Program

Although the effects of cultivation on soil organic matter and nutrient supply capacity are well understood, relatively little work has been done on the long‐term recovery of soils from cultivation. We sampled soils from 12 locations within the Pawnee National Grasslands of northeastern Colorado, each having native fields and fields that were historically cultivated but abandoned 50 yr ago. We also sampled fields that had been cultivated for at least 50 yr at 5 of these locations. Our results demonstrated that soil organic matter, silt content, microbial biomass, potentially mineralizable N, and potentially respirable C were significantly lower on cultivated fields than on native fields. Both cultivated and abandoned fields also had significantly lower soil organic matter and silt contents than native fields. Abandoned fields, however, were not significantly different from native fields with respect to microbial biomass, potentially mineralizable N, or respirable C. In addition, we found that the characteristic small‐scale heterogeneity of the shortgrass steppe associated with individuals of the dominant plant, Bouteloua gracilis, had recovered on abandoned fields. Soil beneath plant canopies had an average of 200 g/m2 more C than between‐plant locations. We suggest that 50 yr is an adequate time for recovery of active soil organic matter and nutrient availability, but recovery of total soil organic matter pools is a much slower process. Plant population dynamics may play an important role in the recovery of shortgrass steppe ecosystems from disturbance, such that establishment of perennial grasses determines the rate of organic matter recovery.

Effects of Cultivation and Abandonment on Soil Organic Matter in Northeastern Colorado

AbstractThe influence of climate and soil texture on soil organic matter losses due to cultivation are rarely addressed due to lack of appropriately paired sites. In addition, soil organic matter recovery on previously cultivated fields is not well understood. In this study, we identified seven sites that have native and abandoned fields and five sites that have native, abandoned, and cultivated fields. All sites are in the northeastern Colorado shortgrass steppe, crossing a precipitation range from 320 to 370 mm, and have sand contents from 36 to 67%. Historical cultivation reduced soil C and N across this region by between 16 and 42%. Although variation in native soil C and N at these sites correlates with climate and soil texture, variation among sites in soil losses due to cultivation is not explained by these variables. We used a statistical model and a simulation model to estimate patterns of soil loss across sites; neither model predicted variation among sites adequately (P &gt; 0.05). We suggest that local‐scale variability in organic matter losses due to cultivation are strongly dependent on management practices. With a simulation model and the data from native, abandoned, and cultivated fields, we estimated that 25 to 120 g C m−2 have been recovered on the abandoned fields during the past 50 yr. Such rates of recovery are small compared with loss rates due to cultivation. Rates of soil organic matter recovery are constrained by natural pedogenic processes, which cannot reverse disturbance processes at a comparable rate in this semiarid environment.

Nitrogen mineralization in native cultivated and abandoned fields in shortgrass steppe

We conducted a set of in situ incubations to evaluate patterns of N availability among dominant land uses in the shortgrass steppe region of Colorado, USA, and to assess recovery of soil fertility in abandoned fields. Replicated 30 d incubations were performed in 3 sets of native (never cultivated), abandoned (cultivated until 1937), and currently cultivated, fallow fields. Net N mineralization and the percentage of total N that was mineralized increased in the order: native, abandoned, cultivated. Higher soil water content in fallow fields is the most likely reason for greater mineralization in cultivated fields, while higher total organic C and C/N ratios in native and abandoned fields may explain differences in mineralization between these land uses. Recovery of soil organic matter in abandoned fields appears to involve accumulation of soil C and N under perennial plants, but probable methodological artifacts complicate evaluation of the role of individual plants in recovery of N availability. Higher N mineralization and turnover in cultivated fields may make them more susceptible to N losses; recovery of N cycling in abandoned fields appears to involve a return to slower N turnover and tighter N cycling similar to native shortgrass steppe.