Effects Of Fertilization On Soil C, N & P Pools And Its Involved Enzyme Activity In Paddy Wetland Ecosystems

Fertilization affects surrounding environment. The transformation of paddy soil carbon, nitrogen, and phosphorous is influenced by fertilization, which thus impacts the surrounding water body, atmosphere, and so on. Two field fertilization experiments, one for 27 years and the other for 6 years, were selected as study materials. The main objectives of this research were:(1) to study soil organic carbon (SOC) sequestration, the changes of soil total nitrogen (STN), soil mineral nitrogen (SMN), soil total phosphorous (STP), and Olsen-P in the profile; (2) to investigate the distribution of soil urease activity and neutral phosphatase activity in the profile; and (3) to determine the concentration of N and P in paddy water under different fertilization. The followings were the main results.1. Fertilization influenced SOC content in the plow layer and SOC density in the top 0-100 cm depth soil layer. At Nanchang site after 27 years experiment, fertilization increased SOC content in the plow layer compared to CK. At Jiaxing site after 6 years experiment, urea influence SOC content in the plow layer gently, and a low rate of urea depleted SOC; pig manure (PM) markedly increased SOC content in the plow layer. At Nanchang site, SOC density in the 0-100 cm depth soil layer ranged from 73.1 to 91.4 Mg C ha-1. Soil organic C density of all the fertilizer treatments was greater than that of the CK. Those treatments that combined both chemical fertilizers and organic amendments had greater SOC densities compared to those only receiving chemical fertilizers. At Jiaxing site, SOC density in the 0-100 cm depth soil layer varied from 120.0 to 125.1 Mg C ha-1 in the urea treatments, and from 121.7 to 133.0 Mg C ha-1 in the PM treatments. Urea was not beneficial to sequester SOC. Soil organic C density increased with increasing rate of PM. At two experimental sites, SOC density was closely correlated to the input C from organic amendments. Carbon sequestration in paddy soils could be obtained by fertilization with organic amendments. Fertilization combined both chemical fertilizers and organic amendments is an effective sustainable practice to sequester SOC. 2. Fertilization affected STN and SMN in the plow layer, but not under the plow layer. There were significant correlations among soil total N, SMN, NH4+-N and NO3--N. At Nanchang site, STN of treatments with fertilizers was greater than that of the CK without fertilizer. Those treatments that combined both chemical fertilizers and organic amendments had greater increasing effects on STN compared to those only receiving chemical fertilizers. At Jiaxing site, urea gently affected STN in the plow layer, and a low rate of urea depleted STN; PM significantly sequestered STN. The change trend of SMN was similar to STN. Fertilization with organic amendments can maintain paddy soil N.3. Phosphorus fertilization increased STP and Olsen-P in the plow layer but not under the plow layer compared to P-free fertilization. Downward movement of P was not observed. At Nanchang site, STP and Olsen-P in the plow layer was exhausted more in the NK than in the CK. Those treatments that combined both chemical fertilizers and organic amendments had greater increasing effects on STP and Olsen-P compared to those only receiving chemical fertilizers. At Jiaxing site, superphosphate and PM significantly increased STP and Olsen-P in the plow layer, and the effect increased with increasing P rates.4. Fertilization influenced soil urease activity in the plow layer but not under the plow layer. Soil urease activity decreased with increasing soil depth. Soil urease activity was positively and significantly correlated with STN, SMN, NH4+-N, and NO3--N. At Nanchang site, soil urease activity of treatments with fertilizers was greater than that of the CK without fertilizer. Those treatments that combined both chemical fertilizers and organic amendments had greater increasing effects on soil urease activity compared to those only receiving chemical fertilizers. At Jiaxing site, urea increased soil urease activity in the plow layer, and an excessive rate of urea inhibited soil urease activity; PM had greater increasing effects on soil urease activity compared to urea. Fertilization with organic amendments increased soil urease activity in the plow layer.5. Fertilization affected soil neutral phosphatase activity in the plow layer but not under the plow layer. Soil neutral phosphatase activity decreased with increasing soil depth. Soil neutral phosphatase activity was positively and significantly correlated with STP and Olsen-P. At Nanchang site, soil neutral phosphatase activity of the treatments with fertilizers was greater than that of the CK without fertilizer, but except PK treatment. Those treatments that combined both chemical fertilizers and organic amendments had greater increasing effects on soil neutral phosphatase activity compared to those only receiving chemical fertilizers. At Jiaxing site, superphosphate decreased soil neutral phosphatase activity in the plow layer; PM increased soil neutral phosphatase activity in the plow layer. Fertilization with organic amendments increased soil neutral phosphatase activity in the plow layer.6. At the end of rice season, fertilization affected N and P concentration in paddy surface water but not in paddy groundwater at a 100-cm depth. The N and P concentration in paddy water were significantly and positively correlated to soil N and P, soil urease activity, and neutral phosphatase activity in the corresponding soil layer. Soil urease activity and neutral phosphatase activity contributed to the loss potential of paddy N and P. At Nanchang site at the end of rice season, fertilization increased N and P concentration in paddy surface water. Those treatments that combined both chemical fertilizers and organic amendments had greater increasing effects on N and P concentration in paddy surface water compared to those only receiving chemical fertilizers, but NO3--N concentration in paddy surface water decreased with increasing rate of organic amendments. At Jiaxing site at the end of rice season, urea increased N concentration in paddy surface water, but a low rate of urea decreased N concentration in paddy surface water; PM increased N concentration in paddy surface water, but N03--N concentration in paddy surface water decreased with increasing PM rate. At Jiaxing site at the end of rice season, both superphosphate and PM markedly increased P concentration in paddy surface water.7. Paddy fields are sustainable wetland ecosystems. The paddy wetland ecosystem may act as a sink for C, N, and P through plants and soil apart from being a pollution source due to the existence of barriers and plow pans. The plants and soil are the reservoirs for nutrients in the paddy system, barriers on the paddy perimeter prevent runoff, and plow pans prevent leaching. Paddy wetland ecosystem can act as a sink especially for organic amendments, which could partly or totally substitute for chemical fertilizers, with proper management such as reduced-drainage or zero-drainage taking the advantage of the barriers. If with improper management, paddy fields may become non-point pollution sources.