Abstract
The variations of soil organic carbon and its fractions in different paddy-upland multiple cropping rotation systems were evaluated in field trials in two consecutive years. During paddy-upland multiple cropping rotations conducted over 2 years, the content of soil total organic carbon (TOC) first increased and then decreased. The content of readily oxidized organic carbon (ROC) was highest at the rice tillering stage and lowest at the mature stage. The soil microbial biomass carbon (SMBC) was highest at the tillering stage. The dissolved organic carbon (DOC) content was highest at the mature stage. The maximum and minimum differences were at the booting stage and mature stage for TOC content, at the greening stage and booting stage for ROC content, at the mature stage and the greening stage for DOC content, at the tillering stage and the greening stage for SMBC, respectively. The soil TOC and DOC contents showed the largest variations in the 'winter fallow-early rice-late rice → winter fallow-early rice-late rice' rotation. The soil ROC content showed the largest variation in the 'milk vetch-early rice-late rice → rape-peanut-late rice' rotation. The maximum variation of SMBC was in the 'vegetables-peanut/corn-late rice → milk vetch-early rice-late rice' rotation. The 'potato-maize/soybean-late rice → vegetables-peanut/corn-late rice' rotation resulted in higher soil TOC content at the booting stage. The pattern of 'milk vetch-early rice-late rice → oil rape-peanut-late rice' led to higher soil ROC contents in the early and middle growth stages of late rice. In the 'rape-peanut-late rice → potato-maize/soybean-late rice' rotation, the highest DOC contents were at the greening stage and the mature stage, and the highest SMBC were at the booting stage and the heading stage, respectively. All these diffe-rences were significant. The rank the contents of soil organic carbon fractions from highest to lowest followed the order: TOC>ROC>SMBC>DOC. The results suggested that paddy-upland multiple cropping rotation systems could increase the contents of soil organic carbon and its fractions and improve soil quality and fertility.
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