Objectives Straw C/N ratio affects its nutrient release and use. Suitable application of nitrogen fertilizer would help creating advantageous microbial properties for the use of straw.
Methods A field cultivation experiment was carried out in the Red Soil Experimental Station in Hengyang, Hunan province. In the field experiment, 200 g of red soil (air dried) was added separately with blank (CK), urea 0.157 g (N1), urea 0.939 g (N2), maize straw 9 g (S), maize straw with 0.157 g urea (SN1), and maize straw with 0.939 g urea (SN2), packed in nylon bag and buried into soil. The C/N ratios of the treatments S, SN1 and SN2 were 53∶1, 37∶1 and 15∶1, respectively. Soil samples were collected from the nylon bags at the 7, 14, 21, 28, 49 and 150 days of incubation. Soil organic carbon (SOC), soil microbial biomass carbon (SMBC), soil microbial biomass nitrogen (SMBN), soil microbial entropy (SMBC/SOC) and soil microbial carbon nitrogen ratio (SMBC/SMBN) were determined.
Results The SMBC and SMBN contents were the highest at 21st day and the 14th day, respectively. Compared with the CK, the SMBC and SMBN in the treatments with straw were increased by 4–5 times and 6–8 times at the 150 th day. For the six sampling, the SMBC in the three straw containing treatments (S, SN1 and SN2) were averaged 1425, 1379 and 1462 mg/kg, respectively, about 10 times of the other treatments; their SMBN were averaged 172, 181 and 193 mg/kg, respectively, about 8 times of the other treatments; the microbial entropy were 3.57, 3.41 and 3.57, respectively, about 2.8 times as much as the other treatments; and there were no significantly differences in the SMBC, SMBN and microbial entropy among the three straw containing treatments. The SMBC/SMBN ratio of N1, N2 treatment was significantly lower than S treatment. The differences in SMBC/SMBN ratio were not significant among straw containing treatments in the first 28 days. After 28 days, the SMBC/SMBN ratio in S treatment was significantly higher than those in SN1 and SN2 treatments, and at the 150 th day, the SMBC/SMBN ratio in S treatment was about 10, while those in the SN1 and SN2 were nearly 6.
Conclusions Straw addition in red soil could significantly increase SMBC, SMBN and microbial entropy. Both straw and nitrogen addition will reduce the SMBC/SMBN ratios. The regulation of C/N at 37∶1 could not meet the nitrogen requirement for microbial decomposition when straws are added into soil. Therefore, returning maize straw requires more nitrogen input for advantageous soil microbial properties in southern red soil.
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