Abstract
In southern Xinjiang province, northwest China, farmland is undergoing rapid conversion to orchards or agroforestry. This has improved land-use efficiency but has also caused drastic ecological changes in this region. This study investigated the effects of farmland conversion to orchard or agroforestry on soil total organic carbon (TOC) and several soil labile fractions: readily oxidizable carbon (ROC), light fraction organic carbon (LFOC), and dissolved organic carbon (DOC). Soil samples were collected from seven cropping treatments: a monocultured wheat field (Mono), a 5-year-old jujube orchard (5 J), a 5-year-old jujube/wheat alley cropping system (5 JW), a 10-year-old jujube orchard (10 J), a 10-year-old jujube/wheat alley cropping system (10 JW), a 15-year-old jujube orchard (15 J), and a 15-year-old jujube/wheat alley cropping system (15 JW). The results show that the ROC concentrations varied from 0.17 ± 0.09 g/kg to 2.35 ± 0.05 g/kg across all land-use types and soil depths studied. It was higher in the 0–10 cm and 10–20 cm layers of treatment 10 JW than in other treatments and significantly greater than in the Mono treatment. The highest value of DOC was reached at 593.04 mg/kg in the 15 JW treatment at 0–10 cm. Labile organic carbon decreased with increasing depth in all treatments. The proportion of ROC and LFOC to TOC decreased with increasing soil depth. In all treatments, the ratio of DOC to TOC generally decreased initially and then increased again with increasing depth. Correlation analysis showed that ROC, LFOC, and DOC were closely correlated with TOC (p < 0.01). The ROC, LFOC, and DOC concentrations were significantly correlated with each other (p < 0.01). Following conversion of farmland to jujube orchard or agroforestry, the content and activity of soil organic carbon tended to increase due to augmentation of plant residues. Thus, jujube orchards and agroforestry systems are effective methods to restore soil organic carbon.
Highlights
The soil carbon pool is the largest pool of actively cycling C in terrestrial ecosystems and plays a key role in the global carbon cycle and soil microbial activities [1,2]
The total organic carbon (TOC) content in the soil of the different land-use types rapidly decreased with greater soil depth, and the effects of land use on TOC were most pronounced in the topsoil (0–30 cm) (Figure 2)
With increasing age of the jujube orchard, TOC content increased through the entire 0–100 cm soil profile
Summary
The soil carbon pool is the largest pool of actively cycling C in terrestrial ecosystems and plays a key role in the global carbon cycle and soil microbial activities [1,2]. Fractions in the soil are characterized by fast movement, instability, easy oxidation, and decomposition. These fractions can serve as excellent indicators of soil quality as influenced by land-use/cover changes and human activities and can be used to characterize short-term changes in soil organic carbon [7,8,9,10,11]. Land-use conversion can rapidly affect the labile organic carbon fractions of soil and affects the nutrient cycling and biomass production [12,13,14,15,16,17]. The readily oxidizable organic carbon fraction (ROC) responds most rapidly to changes in tillage and stubble treatments and is an indicator of soil quality due to its ease of measurement [18,19,20,21]
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