Dynamics Of Soil Labile Organic Carbon Fractions And C-cycle Enzyme Activities Under Straw Mulch

Straw mulch is one practice for effective disposal that can decrease air pollution and improve soil organic carbon, and also can promote the development of circular agriculture. To explore the impact of different straw management practices on soil organic carbon fractions and related enzyme activities, and the enzymatic mechanism of labile soil organic carbon fractions (LOFC) in Chengdu plain. In this paper, a field experiment was conducted in Dujiangyan City from November 2013 to October 2014, and six treatments were designed including control (no fertilizer or straw), chemical fertilizer only,10% straw nitrogen (N) plus 90% fertilizer N,20% straw N plus 80% fertilizer N,30% straw N plus 70% fertilizer N,100% straw. We main study the concentrations dynamics of soil total organic carbon (TOC), water-soluble organic carbon (WSOC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), permanganate oxidized organic carbon (POXC) and the activities of cellulase, β-glucosidase, polyphenol oxidase and peroxidase; and explore the relationships between LOFC with C-cycle enzymes under straw mulchs. The main research results were as follow:(1) The TOC concentrations in different treatments showed no significant (P>0.05) difference in the short term, but the LOCF concentrations increased in varying degrees after fertilizer. The concentrations of WSOC, DOC and MBC under five fertilizer treatments were 17.9-144.0,-12.3-208.8,1.9-81.1 and 0.3-70.5% higher than those of control under rice-wheat cropping systems. Meanwhile, the concentrations of WSOC, DOC, MBC and POXC under four straw treatments were significantly (P<0.05) higher than those under chemical fertilizer treatment in most wheat and rice growing stages, respectively. Among them, the 30% straw N plus 70% fertilizer N treatment was the optimum straw return amount to enhance LOCF concentrations because the WSOC, DOC, MBC and POXC were 26.9-75.1,14.9-79.1,15.7-47.6 and 1.7-34.4% higher than those under chemical fertilizer only treatment, respectively; and its WSOC, DOC, MBC, POXC concentrations increased 65.2-257.4,1.4-264.2,33.4-187.6 and 0.0-65.4% compare with before treatment, respectively.(2) Straw treatments can improve soil enzyme activities in varying degrees, in which their cellulase, P-glucosidase, polyphenol oxidase and peroxidase activities were 1.08-2.06, 1.01-1.94,1.04-2.47 and 0.96-2.08 times to control; and were 1.00-1.69,0.90-1.77, 0.98-1.80 and 0.96-1.86 times to chemical fertilizer only, respectively. Among them, the 30% straw N plus 70% fertilizer N and 20% straw N plus 80% fertilizer N treatments showed the most obvious effects. The cellulose activities of 30% straw N plus 70% fertilizer N treatment were significantly (P<0.05) higher than those of 10% straw N plus 90% fertilizer N treatment in most of wheat growth stages, and their β-glucosidase, polyphenol oxidase and peroxidase activities were significantly (P<0.05) higher than those of 100% straw treatment in most of rice growth stages. The cellulase, β-glucosidase, polyphenol oxidase and peroxidase activities under 20% straw N plus 80% fertilizer N treatment were also significantly (P<0.05) higher than those of 10% straw N plus 90% fertilizer N and 100% straw treatments in most of crop growth stages.(3) LOCF showed significant positive correlation with TOC, in which MBC and TOC had the highest correlation coefficients. The correlation coefficients between MBC and TOC in 0-20 and 20-40 cm soil layers were 0.804 and 0.674 (P<0.05) in wheat growth stages, respectively; and were 0.699 and 0.677 (P<0.05) in rice growth stages, respectively. Moreover, the correlation coefficients between MBC and other fractions were higher than those of other fractions (except for the correlation coefficient between WSOC with DOC). Therefore, MBC can be used as the most sensitive indicator to indicative TOC changes.(4) Soil C-cycle enzymes were closely related to the conversion or formation of soil organic carbon. Cellulase and/or P-glucosidase were positive impact factors for WSOC and DOC (P<0.05); and cellulose also was positive impact factor for POXC (P<0.05). Polyphenol oxidase was negative impact in 0-20 cm soil layer of wheat growth stages, but the positive impact factor for WSOC and DOC in wetland system of rice growth stages (P<0.05). Peroxidase was the negative factor for WSOC, DOC and POXC (P<0.05).