Variation tendency of organic sulfur and aryl sulfatase activities under long-term different fertilization in purple soil

Objectives The effect of long-term fertilization on the total and organic sulfur contents and their change trends were studied to provide theoretical basis for reasonable sulfur supply and fertilizer application.

Methods The study was based on a long-term experiment, which was located at the National Purple Soil Fertilizer and Fertilizer Efficiency Monitoring base of Southwest University, Beibei District, Chongqing, and the crop system was rice‒wheat rotation every year. The experiment was started in 1991 and had been carried out continuously for 22 years until the time of sampling. Seven treatments were selected for the sampling, including: non-fertilization (CK), chemical fertilizers including K₂SO₄ (NPK_s), pure manure (M), NPK_s plus manure (MNPK_s), NPK_s plus straw returning (SNPK_s), application of nitrogen (ammonium chloride), phosphorus, potassium (potassium chloride) and straw returning (SNPK_Cl), fallow all the time (F). Depth of 0‒20 cm soil samples were collected to determine the contents of total S (TS), organic S (OS) and three fractions of organic S (C-bond, ester-bond and residual S) and the activities of aryl sulfate esterase.

Results 1) Compared with the initial soil in 1991, the contents of TS and OS in treatments except CK were increased significantly. Except M and NPK_s, the contents of TS varied significantly among the treatments and in descent order of MNPK_s > SNPK_s > M ≈ NPK_s > F> SNPK_Cl. The proportion of OS in TS also changed. The content of OS was decreased significantly in CK treatment, and increased significantly in other treatments (P < 0.05). The highest increase was in MNPK_s and SNPK_s, however, the proportion of OS in TS was only 73.3% in MNPK_s while was 92.2% in SNPK_s. Moreover, the defference of OS contents between M and F treatment was not significant, but both were significantly higher than those in SNPK_Cl and NPK_s treatment (P < 0.05). Although the content of OS in SNPK_Cl treatment was significantly lower than those in the former four treatments, the proportion was equivalent to that in SNPK_s and higher than those in others. The content of OS in NPK_s treatment was higher than those in CK and the initial soil, but significantly lower than those in fallow land and M-containing treatments (P < 0.05). 2) Compared with CK, the content of C-, ester- and residual-S in 0‒20 cm soil layer treated with fertilizntion measures were all increased, and the differences among fertilization treatments were significant. Compared with the initial soil, the contents of C- and ester-bond S in all the other treatments increased. The highest change of C-S was in MNPK_s treatment, with an increase of 31.1 mg/kg. The highest change of ester-S was in SNPK_s, with an increase of 37.05 mg/kg. The change of residual-S in the treatments were different among the treatments. That in MNPK_s was increased by 52.2 mg/kg and in F by 11.65 mg/kg, while that in CK was decreased by 46.62 mg/kg. 3) The activity of aryl sulfate esterase was merely decreased by 8.19 μg/(mL·h) in SNPK_Cl, and by 7.80 μg/(mL·h) in F while in the others were all increased to different degrees, and the largest increase was 37.77 μg/(mL·h) in SNPK_s, followed by M treatment with an increase of 33.91 μg/(mL·h).

Conclusions Long-term application of pure chemical fertilizer, pure organic fertilizer and both two fertilizers could increase the content of C- and ester-S in purple soil, decrease that of residue-S, so increase the bioavailability of S. No fertilization would decrease the bioavailability of soil organic S. The application of chlorine-containing chemical fertilizer is effective in increasing availability of soil S when combining applied with organic materials, but its long-term application seems to decrease the activity of aryl sulfatase. Therefore, the application of chlorine-containing chemical fertilizer should be paid close attention as its effect on soil pH in the long run.