Driving factors of organic sulfur mineralization in brown soil under long-term fertilization

Objectives We assessed the effects of long-term fertilization on soil organic sulfur mineralization, enzyme activities and the main driving factors of organic sulfur mineralization in maize-soybean rotation system in a brown soil.

Methods The experiment with 15 treatments in brown soil was established by Shenyang Agricultural University in 1979, the cropping system was maize-maize-soybean rotation. The seven treatments used in this study were CK (no fertilization), N₁ (low-level chemical N fertilizer), N₂ (high-level chemical N fertilizer), N₁P (low-chemical N+P fertilizer), N₁PK (low-chemical N+PK fertilizer), M₁ (low-level pig manure) and M₁N₁PK (low-level pig manure+low-chemical N+PK fertilizer). Soil samples (0–20 cm) were collected in 2014 and 2015 to determine soil basic physical and chemical property (pH, soil organic carbon and total nitrogen), organic S mineralization rate, soil neutral protease and soil aryl sulfatase activities. A chamber culture experiment was also conducted to assess the mineralization characteristics of organic S at different temperatures.

Results Long-term fertilization increased the mineralization of soil organic sulfur and the overall change trend was M₁N₁PK > M₁ > N₁PK, N₁, N₂ > N₁P > CK. The mineralization of organic sulfur did not change with the increase of chemical nitrogen fertilizer application rate. Compared with no fertilization, application of pig manure or combined application of pig manure and chemical fertilizer significantly increased soil organic sulfur mineralization. According to the first-order kinetic equation fitting, the long-term fertilization increased the organic sulfur mineralization potential. Environmental temperature and crop species significantly affected the mineralization amount and mineralization potential of organic sulfur. The mineralization amount and potential of organic S increased significantly with the increase of environmental temperature. The mineralization amount and mineralization potential of organic sulfur in maize planting year were higher than those in soybean planting year. Compared with the application of chemical fertilizer alone, M₁N₁PK treatment increased the activities of neutral protease and aryl sulfatase by 96%–220% and 264%–986%, respectively. Total mineralization of organic sulfur was positively correlated with soil organic carbon (r = 0.7693) and total nitrogen (r = 0.7554) in maize planting year.

Conclusions Organic sulfur mineralization in brown soil was affected by fertilizer, temperature and crop species. The activities of soil aryl sulfatase and neutral protease as well as the mineralization potential and mineralization amount of organic sulfur were improved by combining application of organic and inorganic fertilizers. The organic carbon and total nitrogen in the soil were the main driving forces of organic sulfur mineralization in the maize season. Maize planting could promote the mineralization of organic S as the relatively large biomass.