Crop cultivation and intensive grazing affect organic C pools and aggregate stability in arid grassland soil

X-G. Li, Z-F. Wang, Qifu Ma, F-M. Li

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    The effects of cultivation and overgrazing on soil quality in and regions have been rarely addressed. This study investigated the roles of cropping and grazing in soil organic C pools and aggregate stability at 0-20 cm depth by comparing conventional grazing (non-fenced ever), intensive grazing (fenced for 22 years) and cropping (cultivated for 40 years) in the and Hexi Coniclor of northwestern China. Total soil organic C (TOC) under non-fenced grazing was 21.6 g kg(-1) (or 52.9 Mg ha(-1)), which was 19.9% (or 13.2% mass per area) lower than that under fenced grazing, because of lower stable organic C fraction (<0.05 mm) (i.e., 15.2 g kg(-1) or 37.4 Mg ha(-1) in non-fenced versus 19.2 g kg(-1) or 44.5 Mg ha(-1) in fenced grazing). Cropping had similar TOC concentration but 15.7% less TOC mass per area compared with non-fenced grazing mainly due to a decrease in coarse organic C (2-0.1 mm) (i.e., 4.1 g kg(-1) or 10. 1 Mg ha(-1) in non-fenced versus 2.9 g kg(-1) or 6.0 Mg ha(-1) in cropping). Non-fenced grazing produced 1.49, 1.17 and 0. 19 g kg(-1) of soil carbohydrate C extracted by concentrated acid, diluted acid and hot water, respectively. The three carbohydrate C fractions were increased by 21.5, 14.5 and 15.8% under fenced grazing but lowered by 12.8, 18.8 and 21.1 % under cropping, respectively. Soil mineralized C after 51 -day incubation was the highest under fenced grazing followed by non-fenced grazing, and the lowest under cropping. Percentage of water-stable aggregates (>0.25 mm) in total aggregates and mean weight diameter were 15% and 0.28 rum under cropping, significantly lower than 65% and 3.11 mm under non-fenced grazing and 65% and 2.84 mm under fenced grazing. The aggregates of > 1 mm were almost entirely demolished under cropping when subjected to wet sieving. Reduction of soil carbohydrates under cropping was closely related to the decline in aggregate water-stability. The negative effects of cropping on soil organic C pool and aggregate water-stability may suggest that cropping on this and grassland is not sustainable unless no-tillage is adopted. In favor of increasing soil carbohydrates and maintaining soil aggregation, fencedgrazing would be a better option than cropping and non-fenced grazing for the management of and grasslands. (C) 2007 Elsevier B.V All rights reserved.
    Original languageEnglish
    Pages (from-to)172-181
    JournalSoil & Tillage Research
    Issue number1-2
    Publication statusPublished - 2007


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