Search

Aims: Cropland revegetation is an effective measure to curb soil erosion on eroding hillslopes and increase farmers' income at depositional check dams. However, how soil microbial metabolic limitation responds to cropland revegetation in erosion and deposition landscapes remains poorly understood, which has substantial implications for carbon (C) retention and nutrient cycling in the eroding environment. Methods: We sampled 0–2 m soils in cropland and revegetated forest and grassland at upslopes and check dams in the hilly-gully region of the Loess Plateau, China. The activities of soil C-, nitrogen (N)-, and phosphorus (P)-acquiring enzymes were analyzed. The improved enzyme vector model (V-T model) based on balance points was used to quantify microbial metabolic limitations based on ecoenzymatic stoichiometry. Results: Microorganisms suffered from no energy (C) limitation, but the relative microbial C limitation was greater in revegetated forest than in cropland. At upslopes, the revegetated forest was primarily limited by P, while the revegetated grassland and cropland were limited by N, which was indicated by the VTN/P limitation values of 4.35, -2.74, and − 1.87, respectively. Microbial P limitation was greater in deep soils of revegetated forest due to the assimilation of P by the root system. At check dams, land-use change had no significant influence on microbial N/P limitations owing to abundant soil C and nutrients and a wet environment. Conclusion: Concludingly, cropland revegetation had a weaker influence on microbial metabolic limitation at the lower-lying topography, which compensates for the current understanding of resource restrictions on microorganisms at slopes or flat areas. [ABSTRACT FROM AUTHOR]

Soil erosion is a major form of land degradation, especially in agroecosystems, which has been effectively controlled by vegetation restoration. However, the interactive role of erosion and cropland revegetation on soil enzyme activities and microbial nutrient limitations is less understood. To address this issue, we examined carbon (C), nitrogen (N), and phosphorus (P) in bulk soils and microbial biomass, enzyme activities, and microbial nutrient limitations in the 0–200 cm soils in eroded and deposited landscapes occupied by cropland, revegetated forest, and grassland. The results showed that the activities of C-, N-, and P-acquiring enzymes were larger in the deposited landscape than in the eroded landscape for 0–20 cm soils in forest and grassland but not in cropland. Microbial metabolism was co-limited by N and P, and the threshold element ratio (TERL) indicated that P was the most limiting factor. Microbial N limitation was lower in the deposited than the eroded zone, especially in surface soils in revegetated forest and grassland. The TERL value was larger at the deposited than at the eroded zone, and a greater difference was found in the surface soils of forest and grassland. Microbial nutrient limitations were mostly explained by C/P and N/P. Conclusively, the deposited areas were characterized by ameliorated enzyme activities, decreased microbial N limitation but relatively strengthened microbial P limitation compared to the eroded area, and such variations existed in the revegetated forest and grassland but not in the cropland, which thus contributes to a better understanding of C and nutrient cycling for agroecosystems and revegetation ecosystems in eroded environments. [ABSTRACT FROM AUTHOR]

• We investigated soil physical and hydraulic properties across a cultivation chronosequence. • A greater decrease in aggregate stability was found in cropland after long-term cultivation. • Long-term cultivation partially improved deep soil bulk density, porosity, field capacity and K s. • The parameters of soil water retention curve were unaffected by the cultivation time. Black soils (Mollisols) are inherently productive and fertile but are deteriorating due to long-term intensive cultivation. Understanding the changes in soil physical and hydraulic properties in topsoils and deep soils after converting forest to cropland would reveal the mechanism related to the black soil degradation. Herein, we investigated the variations in the soil water-stable aggregates, soil water content, bulk density, field capacity, total porosity, saturated hydraulic conductivity, and parameters of soil water retention curve at the 0–100 cm depth in four sampling fields. These fields included a forest as control and three croplands that were converted from forest 41, 50 and 65 years ago in the black soil region of Northeast China. We showed that converting forest to cropland decreased >2 mm aggregates and aggregate stability especially at 0–15 and 70–100 cm depths, with a greater decrease in cropland after relatively longer time of cultivation. Conversion of forest to cropland significantly decreased the soil water content at the 0–15 cm depth but increased it at the 15–100 cm depth, and the increase in the 50–100 cm soil layer was strengthened with cultivation time. The decreases in field capacity, total porosity and saturated hydraulic conductivity and increase in bulk density at the 0–50 cm depth following the conversion of forest to cropland were higher after 41 and 50 years of cultivation than those after 65 years of cultivation. However, changes in these metrics at the 50–100 cm depth increased with cultivation time. The parameters (i.e., θ r , θ s , α and n) of the soil water retention curve were not affected by the cultivation time after converting forest to cropland. These results highlighted that long-term cultivation after the conversion of forest to cropland resulted in the changes in soil physical and hydraulic properties in the 0–100 cm soil profiles, and thus, the response of deep soils should not be ignored in this black soil region. [ABSTRACT FROM AUTHOR]

Soil carbon (C) and nitrogen (N) combining with physical and chemical properties play important roles in terrestrial C and nutrient cycling; however, how C and N in deep soils respond to land-use change is less understood, hindering the precise evaluation of soil C dynamics and budgets. Here, we present results regarding soil C, N and related soil properties at 0–500 cm depth as affected by afforestation at two sites (Fufeng and Yongshou) with contrasting soil inorganic C (SIC) levels in China's Loess Plateau. The concentrations of SIC ranged from 12.94 to 20.17 g kg−1 in Fufeng and from 0.18 to 0.36 g kg−1 in Yongshou. The objectives of this study were to examine how soil C and N in deep soils are altered following afforestation and how these effects vary with soil depth and site. We found that the establishment of woodland on cropland increased soil porosity and saturated hydraulic conductivity but decreased the bulk density in the topsoils (0–20 cm). The conversion of cropland to woodland resulted in an increase of soil electric conductivity in the topsoils and a decrease in the deep soils in Fufeng but had an opposite influence with soil depth in Yongshou. This land-use change increased organic C and total N concentrations along the 0–500 cm soils at both sites but had divergent effects on the concentrations of SIC and soil total C (STC) between the two sites. Overall, the concentrations of SIC and STC increased in Yongshou but decreased in Fufeng after woodland establishment. The results from this study indicated the determination of the initial SIC concentration on the response of C to afforestation in semiarid soils, and such determination should be considered in biogeochemical models. • Afforestation decreased pH and increased SOC and TN regardless of sites. • Effects of afforestation on soil C were dependent on initial SIC level. • Afforestation increased SIC and STC in low SIC site and reduced them in high SIC site. [ABSTRACT FROM AUTHOR]

Cultivation after the converting natural ecosystems to agricultural land is often accompanied by accelerated soil erosion. To better understand how land-use conversion and agricultural erosion separately affect aggregate-associated nitrogen (N) and organic carbon (OC), we examined the responses of aggregate-associated N and OC to both the converting forest to agricultural land and erosion in the agricultural region of Northeast China. Non-erosion sites in forest and agricultural land converted from forestland 60 years ago were selected to address the effects of land-use change. The sites with various erosion intensities in an eroding area of sloping cropland (i.e., non-erosion, light erosion, moderate erosion and heavy erosion) were selected to compare the effects of the erosion intensity. The measured soil metrics included the soil bulk density, water-stable aggregates, and aggregate-associated OC and N along 0–70 cm soil profiles. Converting forest to agricultural land did not significantly influence the OC and N concentrations in each aggregate, while the land-use change significantly reduced the OC and N stocks in macroaggregates but increased them in microaggregates and silt+clay fraction. However, erosion significantly decreased the OC and N concentrations and stocks in each aggregate. The changes in the OC and N stocks in aggregates were mainly due to the depletion of soil aggregates in responding to the conversion of forest to agricultural land but due to decreases in OC and N concentrations in aggregates in responding to soil erosion. Our results indicated that the effects of erosion on aggregate-associated OC and N were greater compared with the effects of converting forest to agricultural land in this Mollisol region. • Land-use change didn't significantly affect OC and N concentrations in aggregates. • Erosion decreased OC and N concentrations in aggregates. • OC and N stocks in aggregates were determined by their concentrations during erosion. • OC and N stocks in aggregates were dominated by aggregates mass following land-use change. [ABSTRACT FROM AUTHOR]