| The process of the migration and transformation of carbon (C), nitrogen (N) and phosphorus (P) in agricultural ecosystems and its involved net results (i.e."source" or "sink") are closely related to keep the ecosystem healthy and maintain its ecological function. At present, the N and P losses from cropland have made a remarkable contribution to the deterioration of water quality (e.g. eutrophication). In China, low utilization of water and fertilizers caused by the excessive fertilizer application and unreasonable water and fertilizer managements is the main reason leading to the huge loss of N and P from agricultural fields. In this research, field plot experiments were conducted to study the migration and transformation of C, N and P in rice paddy ecosystems in response to two irrigation regimes (continuous flooding irrigation, CF; alternate wetting and drying irrigation, AWD) and four N managements (zero N control, NO; conventional urea, UREA; controlled-release bulk blending fertilizer, BBF; polymer-coated urea, PCU) in the Tiaoxi River watershed, Taihu Lake region in2010and2011. The main contents of this research included:(1) the change characteristics of N&P concentrations as well as N&P losses via surface runoff and leaching from paddy fields;(2) the growth, grain yield, and water and N use efficiencies of late-season rice:(3) the C, N and P accumulation and partitioning and C:N:P stoichiometry in rice plant; and (4) the nutrient contents and microbial properties of paddy soils. Results of this study would provide the scientific basis for efficient use of water and fertilizers to reduce water consumption, fertilizer use, and N&P losses in rice production systems. The following were the main results.1. The average pH of surface and percolation water ranged from6.0to8.4and5.8to6.7, respectively. The water pH was affected by the combined effects of N fertilization, rainfall and soil pH. For surface water, the TN, NH4+-N and NO3--N concentrations reached the maximum at the first day after fertilization in UREA and BBF, while TN and NH4+-N concentrations got peak values within the first5days after fertilization in PCU. For percolation water, the peak concentrations of TN, NH4+-N, NO3--N (except PCU) and NO2--N in all treatments were observed within the first10days after fertilization. Inorganic-N was the main form of free nitrogen in both surface and percolation water in paddy fields. NH4+-N was the main form of N which accounted for60%～70%of TN, N03--N accounted for13%～19%of TN, and NO2--N had the lowest proportion (1.3%～2.2%) of TN in surface and percolation water. Compared with CF, AWD increased N concentrations in both surface and percolation water, though the differences were not significant (p>0.05,the same below). Meanwhile, AWD decreased the amounts of irrigation, runoff, and percolation water by28.0%～41.9%,19.1%～57.9%, and14.2%～21.9%, respectively, and thus reduced the runoff and leaching losses of TN by51.6%(2010) and9.4%(2011). N fertilization significantly increased N concentrations in surface and percolation water as well as TN loss via runoff and leaching. Compared with UREA, BBF and PCU significantly decreased N concentrations in surface and percolation water as well as TN loss via runoff and leaching.2. TP and DP concentrations exhibited a similar trend and peaked within the first day after fertilization in surface water, whereas peaked within the first7days after fertilization in percolation water. PP was the main form of P in surface water, whereas DP was the main form of P in percolation water. Compared with CF, AWD decreased TP and DP concentrations slightly but had little impact on DP/TP. Meanwhile, AWD decreased the runoff and leaching losses of TP by24.7%～57.4%and21.0%～25.3%, respectively. N fertilization significantly increased P concentrations in surface and percolation water as well as TP loss via runoff and leaching. Compared with UREA, BBF increased P concentrations in surface and percolation water as well as TP loss via runoff and leaching, whereas PCU decreased these parameters.3. AWD increased plant biomass, grain yield, and water and nitrogen use efficiencies (WUE and NUE) compared with CF. N fertilization also significantly enhanced these parameters compared with zero N control. Similar plant biomass, grain yield, WUE and NUE were observed between BBF and UREA, which were significantly lower than those of PCU. The W x N interactions on plant biomass, grain yield and NUE were not significant, while those on WUE were significant. The combined AWD and PCU treatment increased grain yield and subsequently increased WUE and NUE with reduced water input by AWD and enhanced N utilization by the PCU as compared to conventional water and N managements.4. The C, N and P concentrations, accumulation and partitioning, and C:N:P ratios varied substantially among plant tissues and crop development stages. Water and N managements had considerable effects on those measured parameters after transplanting, but the W x N interactions were generally insignificant. Compared with CF, AWD did not affect the concentrations and accumulation of tissue C and N, but greatly decreased those of tissue P in the late growth period of rice, resulting in unchanged C:N ratio and enhanced N:P and C:P ratios. N fertilization significantly increased tissue N concentration, slightly enhanced tissue P concentration, but did not affect tissue C concentration, leading to a significant increase in tissue N:P ratio but a decrease in C:N and C:P ratios. The C:N:P ratios were highly correlated both within and across plant tissues. Our results suggested that the growth of rice in the Taihu Lake region was co-limited by N and P under conventional fertilization rate.5. The soil pH, soil TN, TP, NH4+-N and NO2--N contents as well as urease activity (UA) and acid phosphate activity (APA) all increased firstly and then decreased during the rice growing season. Soil NO3-N content showed an opposite trend to the above parameters. Soil organic carbon (SOC) decreased firstly, then increased and finally decreased to harvest. At harvest in2011, SOC and TP contents in all treatments and TN contents in N-fertilized treatments increased in different degrees compared with those of initial soil in2010, implying obvious accumulation. But TN contents in NO treatment decreased from2010to2011, showing depletion during this period. Soil inorganic-N accounted for only0.44%～2.27%of soil TN. Soil NH4+-N and NO3--N accounted for80.6%～97.8%and2.1%～19.3%of inorganic-N, respectively. Soil NO2--N accounted for less than0.5%of inorganic-N and was negligible. Compared with CF, AWD decreased soil pH, SOC and NH4+-N contents but increased soil TN, TP, NO3--N and NO2--N contents, and UA and APA as well as soil microbial biomass carbon (MBC), microbial biomass nitrogen (MBN) and microbial biomass phosphorus (MBP). N fertilization significantly decreased the soil TP content but increased all the other measured parameters. The PCU obtained higher soil fertility and better microbiology parameters compared with BBF and UREA.6. The AWD and PCU combination can play a positive role in saving water, promoting rice production, enhancing WUE and NUE for late-season rice, improving soil fertility, and reducing agricultural non-point source pollution. As a suitable agronomic practice for rice production, the new water and N management can be applied widely in the Taihu Lake area. |
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