Abstract Organic carbon (OC) sequestration through soil aggregation is an important aspect of land use change/conversion (LUCC) influencing the terrestrial ecosystem C cycle, although little is known on the changes in aggregate dynamics and their contributions to OC accumulation after LUCC in regions with serious soil erosion. Therefore, bulk soil samples under four land uses (farmland and three vegetated soils converted from farmland 42 years ago: Robinia pseudoacacia [RP42yr], Caragana korshinskii [CK42yr], and abandoned land [AL42yr]) in the Loess Plateau, China, was collected, separated into seven aggregate size fractions, and examined for OC content. Farmland conversion into AL42yr, CK42yr, and RP42yr increased macroaggregate (>2 mm) and mesoaggregate (2–0.25 mm) proportions, mean weight diameter, and geometric mean diameter but decreased microaggregates (0.25–0.053 mm) amount. Bulk soil and aggregates OC content and stock varied with soil depth and land use types but were usually highest in RP42yr. Mesoaggregates contained higher OC content and stock than other aggregates at 0‐ to 20‐cm depth under all land uses. Increases in the OC stocks of mesoaggregates accounted for 46% and 85% of the increase in bulk soil OC stocks at 0‐ to 20‐ and 20‐ to 40‐cm depth, respectively. Thus, soil OC accumulation after LUCC is mainly due to increased OC stock within mesoaggregates, which is further attributed to increased mesoaggregate proportions. Overall, vegetation restoration promotes the physical protection of OC by increasing soil aggregation, being a management option to enhance the C sequestration potential in ecological fragile regions.
The influence of long-term application of pig manure combined with chemical fertilizer (MCF) or chemical fertilizer (CF) on free-living nematodes was evaluated in this study.The application model of fertilizers lasted 14 years in Mollisols, and treatments included MCF, CF and no fertilizer (NF).A total of 26 free-living nematode genera belonging to seven functional guilds were found in maize fields, and the community structure of free-living nematodes was different in MCF, CF and NF.Pig manure increased the abundance of bacterivores, especially those belonging to c-p1 (Ba1) and c-p2 (Ba2) guilds.Channel index (CI) was higher in NF than in MCF and CF, but enrichment index (EI) was higher in MCF and CF compared to NF.The structure index (SI) was highest in NF among three treatments.Total bacterivores, Ba1 and Ba2 guilds were positively correlated to organic C, total N, available N, total P and available P, but fungivores only had correlation with organic C and soil moisture.The SI index was negatively related to organic C, total N, available N, total P and available P. Collectively, these results indicate that the pig manure or chemical fertilizer normally applied to increase soil nutrition also induce negative influence on soil food web structure as reflected from biological aspect, and that soil nematodes can enhance agroecological assessments of changes induced by long-term fertilizer application in maize field in Mollisols.
Abstract Overstory trees and understory herbs are in different niches in the community; however, how their nutrient limitations change following afforestation remains unclear. Therefore, we used plant nutrient resorption and ecological stoichiometry to illustrate the nutrient relationship between plants and soil while exploring the nutrient limitations of different plants during afforestation. Soil and plant samples were collected from farmland and three Robinia pseudoacacia L. (RP) forests that were 17, 26, and 42 yr old. The physical characteristics of the soil and the content and stoichiometric ratios of C, N, and P in the plants, litter, soil microbes, and soil were measured. Additionally, the resorption of N and P in RP and understory herbaceous biomass (UHB) was assessed. Results showed that, at all sites, N resorption (0.45–0.52) and P resorption (0.39–0.49) were approximately equal in RP, while P resorption (0.48–0.65) was significantly higher than N resorption (0.06–0.32) in UHB. With afforestation age, the N/P resorption ratios (1.06–1.25) were not significantly different in RP but declined significantly from 0.50 to 0.14 in UHB. Results suggest that RP is co‐limited by N and P while understory herbs are limited by P. Although the nutrient limitation of RP and UHB may be different, their nutrient relationship with the soil is consistent after afforestation. There was a significant positive relationship for C and N content but a negative relationship of the C/N ratios between soil and plants. These findings deepen the understanding of a linkage between the aboveground and belowground systems and provide evidence that nutrient limitation changes occur in plants of different ecological niches after afforestation in the Loess Plateau.
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