| Nitrogen is the most important element in geobiochemical circulation and its transformation in soil is related to the risk of nitrogen loss and use efficiency of nitrogen fertilizers. Soil physical, chemical and biological properties were affected by agricultural land used. Then, soil nitrogen transformation was changed and the mechanisms for retaining inorganic N in soils was destroyed, which may lead to environmental problems, particularly under subtropical climate with high temperature and plenty rainfall. However, the effects of different agricultural managements on N cycling in soil and the exact process by which the mechanisms for retaining inorganic N in humid subtropical soils are removed after converting natural forestland to agricultural land is not known. In our study, a 15N tracing study was carried out with with a Markov Chain Monte Carlo sampling algorithm to investigate the effects of agricultural managements (e.g. planting age, organic and chemical fertilizers applied, and land used type, et al.) on N transformation in subgtropical region. These results will provide the theory basis for the regulation of nitrogen and how to decrease nitrous oxide emission in subtropical soils.Firstly, the method of Bremner was used to investigate the effects of conversion of woodland to orchard and orchard planting age on soil organic N component in subtropical soils in Eastern China. The results showed that the total N contents in orchard soils increased with planting age within 30 years. The hydrolysable unidentified N, non-acid hydrolysable N, ammonia N, amino acid N and amino sugar N accounted for about 38.9%, 25.7%, 20.5%, 9.9% and 4.9% of the increased total N in soil, respectively. The percentage of non-acid hydrolysable N increased with planting age, while the percentage of hydrolysable unidentified N and ammonia N decreased with planting age. Thus, the sequence of organic-N component in soil changed with planting age.The changing of organic-N due to agricultural management may lead to N cycling changed. Thus, a 15N tracing study was carried out to investigate the effects of conversion of woodland to orchard and orchard planting age on the gross rates of N transformation occurring simultaneously in soils. The results showed that inorganic N supply rate was kept unchanged with the significant increases in soil organic C and N contents after converting woodland into citrus orchard and with orchard age. This phenomenon most probably due to the increase in turnover time of recalcitrant organic-N, which increased with decreasing soil pH (p< 0.01) along with increasing orchard planting age significantly (p< 0.01). The amoA gene copy numbers of archaeal (AOA) and bacterial (AOB) was stimulated by orchard planting and increased with increasing orchard age. The nitrification capacity (defined as the ratio of gross rate of nitrification to total gross rate of mineralization) increased following the Michaelis-Menten equation, sharply in the first 10 years after woodland converted to orchard, and increased continuously but much slowly till 30 years. Due to the increase in nitrification capacity and unchanged in NO3- consumption, ammonium dominance in inorganic N in woodland soil was shifted to nitrate dominance in orchard soils, thereby, the risk of NO3- loss was expected to increase. No change in capacity of biological inorganic N supply verified the local farmers’impression that the nitrogen demands for fruit growth did not change although soil organic N in orchard soils accumulated with orchard planting age.High rates of inorganic and organic fertilizers are applied is the most remarkable feature of agricultural management. So, nitrogen transformation changing with planting age may be due to nitrogen and organic fertilizers applied. In order to verify the relationship between fertilization and nitrogen cycling in soil, in this study, the method of Bremner and 15N tracing study were conducted to test soil organic N component and the potential gross N transformation rates in a long-term (30y) field experiment with different organic fertilizers management practices in a typical rice field in subtropical region of China. This experiment was initialized in 1983 with four fertilizer treatments, including no fertilizer applied (CK), chemical-fertilizers (NPK), chemical fertilizers-plus-cattle manure (NPKM) and chemical fertilizers-plus-straw (NPKS). The results showed that, comparing to no fertilizer (CK) treatment, the concentrations of total N and organic-N in soil increased under long-term fertilization. Compared to the treatment of chemical fertilizer (NPK), the contents of total N in soil increased in the treatments of organic fertilizers applied with chemical fertilizers. However, the increased component of organic-N was different in the different organic fertilizer treatments. About 60.0% of increased organic-N was acid-hydrolysable N in the cattle manure applied with chemical fertilizers (NPKM) treatment, while 71.7% of increased organic-N was non-acid hydrolysable N in the straw applied with chemical fertilizers (NPKS) treatment.Nitrogen transformation in soil was chanaged with organic-N component. The application of NPKM and NPKS significantly stimulated the mineralization of labile organic-N pool and recalcitrant organic-N pool, respectively. The oxidation of recalcitrant organic-N rate was negligible among the four treatments. Therefore, the enhanced inorganic N supply rates (total mineralization + recalcitrant organic-N oxidation) due to NPKM and NPKS application could be attributed to the increasing mineralization of labile organic-N pool and recalcitrant organic-N pool, respectively. The autotrophic nitrification rate was significantly stimulated by NPK and NPKM application, but was not affected by NPKS application. However, both NO3-consumption rates (immobilization of NO3- and dissimilatory NO3- reduction to NH4+) were generally low and were not affected by fertilizer application. Chemical fertilizer applied with cattle manure could increase autotrophic nitrification rate and thus the potential risk of NO3- leaching and runoff. However, the application of chemical fertilizer with straw might be an alternative management practice in increasing inorganic N supply rate and without enhancing the risk of NO3- leaching and runoff.Comprehensively analysed the results showed above, indicated that inorganic N supply rate was increasing with the increases in soil organic C and N contents in paddy soils, while it was kept unchanged in orchard soil. The most different agricultural management between paddy and orchard soil was water management. Thus, it was hypothesized that, under different soil water contents, the effects of fertilizers on nitrogen cycling in soils were different. In order to investigate the effect of water content on N cycling in soils, in this study, soil samples were collected from tobacco growing season and rice growing season in a long-term field experiment with tobacco-rice system, respectively. A 15N tracing incubation study was conducted to identify changes in N dynamics under aerobic (60%WHC) and anaerobic (water-filled, soil:water = 1:1) condition, respectively. The results showed that, nitrogen transformation was significant different between aerobic and anaerobic condition, and there was a better N retained mechanism in soil under anaerobic condition than aerobic condition. Compared with aerobic condition, water-filled soil increased the gross rate of organic N mineralization, enough NH4+ was provided for rice growing, and excess NH4+ may be immobilized to organic N due to higher gross rate of immobilization compared to aerobic condition. Additionally, gross autotrophic nitrification rate was lower under anaerobic condition than that in aerobic condition. However, under anaerobic condition, autotrophic nitrification rate was significantly stimulated by fertilization treatments too. Fortunately, the process of dissimilatory NO3- reduction to NH4+ (DNO3) was stimulated by water-filled condition. Thus, excess NO3- may be reduced to NH4+, and immobilized to organic N by high rate of immobilization under water-filled condition, and then decreased the risk of inorganic N loss by runoff or leaching.In order to test the results sunnarized from incubation study, a field experiment was conducted to investigate the relationship between inorganic N loss and N transformation in the soil with rice-tobacco rotation. The results showed that, in tobacco growing season, NO3- loss through runoff increased linearly with the rate of N fertilizer applied. Nitrate loss was correlated with gross nitrification of NH4+ positively and with NH4+ immobilization negatively. These results implied that not only N input, but also nitrification and immobilization of NH4+ were key factors controlling NO3- loss. NH4+ loss in all year and NO3- loss in rice growing season were not correlated with N transformation in soil.In a word, nitrogen transformation in soil was affected by agricultural managements (e.g. land used changed, chemical and organic fertilizers applied, water managed, and so on). The gross autotrophic nitrification rate was stimulated by the conversion of woodland to agricultural land, chemical fertilizer and cattle manure applied, and then increased the risk of NO3- loss. Under aerobic condition, nitrification and immobilization of NH4+ were key factors controlling NO3- loss. Compared with aerobic condition, there was a better N retained mechanism in paddy soil. |
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