Concurrent rice and crab farming is an important agricultural process in China's northern rice growing area, which allows the complementary use of scarce sources such as water and land. Rice-crab farming developed from the traditional farming mode of raising fish in paddy fields. As sustainable environmentally-friendly agriculture, this farming mode makes use of the symbiotic relationship between rice and crabs to more effectively use materials, reduce chemical fertilizer and pesticide inputs, increase product safety and overall productivity, and decrease the pollutants discharged from paddy fields. In particular, the combination of organic manure and rice-crab farming not only increases rice production, but also improves soil quality and sustainability. Consequently, soil organic carbon and soil enzymes were measured in this study to assess the influence of rice-crab farming and fertilization on soil quality. A long-term fertilization experiment was set up in Panjin, China in 2009 with six treatments:, organic rice-crab production (modes M1, M2, M3 and M4 which correspond to no chemical fertilizer but added pig manure at 48000, 42000, 36000, 30000 kg/hm², respectively); conventional rice-crab mode (MNP with urea added at 375 kg/hm² and pig manure added at 15000 kg/hm²); and rice monoculture (CK with urea added at 750 kg/hm²). All the modes were replicated three times in a randomized block design. Composite surface soil samples (0-20cm) were collected from each plot in April and October 2009 and 2010. The soil samples were then analyzed for total organic carbon (TOC), labile organic carbon (LOC), moderately labile organic carbon (MLOC), highly labile organic carbon (HLOC), the carbon management index (CMI), and soil catalase, urease, invertase and alkaline phosphatase activities.
The results showed that the TOC, LOC, MLOC and HLOC, and CMI were significantly higher in the organic rice-crab modes than in the rice monoculture mode, and this increase was greater with increasing amounts of organic manure. In 2009, the TOC, LOC, MLOC and HLOC, and CMI in mode M1, which had the highest amount of added organic manure, increased by 33.44%, 26.26%, 35.46%, 47.22%, and 25.76%, respectively, compared to the rice monoculture mode. Similarly, the soil catalase, urease, invertase and alkaline phosphatase activities were significantly higher in the organic rice-crab modes when compared with the rice monoculture mode. This indicates that organic rice-crab farming was efficient at restoring soil organic carbon and improving soil microbial activity in China's northern rice area. In 2010, the highest increase in the LOC (10.11%) and MLOC (5.14%) compared to 2009 was found in the moderate organic manure rice-crab mode (M3). However, the most significant increases in the urease and alkaline phosphatase activities, 80.25% and 46.62%, respectively, were observed in the low organic manure rice-crab mode (M4). A similar but less significant trend was also observed between the organic and conventional rice-crab production modes. A correlation analysis between the soil enzyme activities and the labile organic carbon for different modes indicated that TOC, LOC and MLOC are significantly positively correlated with soil catalase, urease, invertase and alkaline phosphatase activities, with correlation coefficients ranging from 0.584 (P<0.05) to 0.940 (P<0.01). Our results suggest that the organic rice-crab production mode can improve the quantity and quality of soil organic carbon and increase soil enzyme activities and soil fertility.