Studies On Nitrogen And Phosphorus Losses Risk In Different Crop And Rotation Farmlands In Shanghai Suburb

Nitrogen and phosphorous losses from farmland have been identified as one of important causative factors for agricultural non-point source pollution. Only if understanding the risk of nitrogen and phosphorus losses from agricultural land, we can effectively control the losses of nitrogen and phosphorus from farmland. Risk of nitrogen and phosphorus losses varies with not only precipitation processes but also site-specific characteristics. There are different crop and rotation types in different site-specific location, time of application, application rate, fertilization pattern, soil available nutrient, cultivation systems, irrigation and drainage conditions etc., can result in different risk of nitrogen and phosphorus losses from different farmlands. Rainfall variability in different seasonal and inter-annual is also very strong. So it is necessary that continuous monitoring for many site-specific farmlands and many crops to study the risk of nitrogen and phosphorus losses from the farmland of different crops types and rotation patterns.To study the risk of nitrogen and phosphorus losses from different crop and rotation farmlands, 43 representative farmlands including differential crop types, fertilization, soil, irrigation and drainage conditions etc., were selected as monitoring site-specific locations in Shanghai suburb area. Soil nutrient content, plant nutrient absorption, nutrient (N, P and K) input and field management by 50 varieties of crops including 216 harvest seasons were monitored from 2003 to 2005. These farmlands represent 5 main crop types: fruit type, greensward type, hydrophily vegetable type, dry vegetable type and field crop type according to cultivate patterns. These farmlands also represent 5 types of planting patterns: the fruit tree monoculture, the vegetable-vegetable rotation, the vegetable-crop rotation, the greensward only and the crop-crop rotation according to rotation patterns. The N, P and K concentrations of ground water and leaching water were also determined in 11 site-specific locations. The main results are as follows:The range of TDP concentrations of ground water and leaching water for 5 types crop farmland were 0.2-2.69 mg·kg^-1. The tendency of TDP concentrations of ground water and leaching water is as follows, fruit type >greensward type > hydrophily vegetable type > dry vegetable type > field crop type. Phosphorus nutrient surpluses of the farmland soil of different crop farmland soil have significant positive correlation with the mean concentrations of TDP in the groundwater and leaching water. P nutrient surpluses of different crop farmland soil are positively correlated with corresponding contents of available Olsen-P in the surface soil (0-30 cm) after harvest. Contents of soil available Olsen-P after harvest are gradually decreased with the depth increasing of soil profile in the farmlands of different crop and different planting patterns, and the accumulations of soil surface (0-30 cm) available Olsen-P in different crop farmlands account for 52.0%-76.6% of those of soil profile (0-90 cm). So P surpluses of farmland will result in the non-balance of farmland soil, then Olsen-P is accumulated in the soil surface, and more Olsen-P in the soil surface can move into the groundwater and leaching water, accordingly increase the risk of phosphorus losses. The range of NO₃^--N, NH₄⁺-N and TDN concentrations of ground water and leaching water for 5 types crop farmland were respectively 0.41-119.81 mg·kg^-1, 0.21-5.40 mg·kg^-1, 2.39-138.51 mg·kg^-1. The concentration of various nitrogen in fruit fields are the highest. Nitrogen nutrient surface balance of the farmland soil are not well correlated with NO3-N and TDN concentration in the groundwater and leaching water, whereas nitrogen surface surpluses of farmland soil are positively correlated with NH₄⁺-N concentration, which may be related to rainfall conditions and the strong mobile NO₃^--N. Contents of soil available NH₄⁺-N, NO₃^--N after harvest are gradually decreased with the depth increasing of soil profile, and the accumulations of soil surface (0-30 cm) available NH₄⁺-N and NO₃^- -N account for 46.6%-78.6% of those of soil profile (0-90 cm). N nutrient surpluses of farmlands of different crop farmlands are positively correlated with corresponding contents of available NH₄⁺-N, NO₃^- -N in the surface soil (0-30 cm) after harvest. The more the N nutrient surpluses of farmland are, the more the concentrations of corresponding available NH₄⁺-N and NO₃^- -N in the farmland are. But NO₃^- -N and NH₄⁺ -N concentrations of ground water and leaching water change indefinitely for the chanciness of precipitation.Different fertilization of different crop and rotation farmlands results in significant differences of nutrient apparent balance. For different crops, N and P (P₂O₅) surplus of hydrophilic vegetables are the most, 1405.3 kg·hm^-2·a^-1 and 744.6 kg·hm^-2·a^-1 respectively, and the fruit and dry vegetables are the second, and grain crops and greensward are the least. N surplus of greensward is only 84.0 kg·hm^-2·a^-1 and P (P₂O₅) deficit of grain crops is 11.0 kg·hm^-2·a^-1. For different planting patterns, N and P (P₂O₅) surplus of the vegetable-vegetable rotation are respectively 763.6 kg·hm^-2·a^-1 and 528.8 kg·hm^-2·a^-1. N, P (P₂O₅) surplus of the fruit tree monoculture are respectively 418.1 kg·hm^-2·a^-1, 380.2 kg·hm^-2·a^-1. N, P (P₂O₅) deficit of the crop-crop rotation are respectively 6.7 kg·hm^-2·a^-1, 84.2 kg·hm^-2·a^-1.During monitoring time, frequency of application, the quantity of fertilizer and the precipitation process of 216 stubble crops in 43 GPS localization farmland were statistically analyzed. Monthly meanly frequency of application in crop farmlands is 0.24-0.75. Monthly meanly frequency of application in hydrophytic vegetable and dry vegetable farmland respectively are 1.6-1.7 times and 1.2-2.2 times than those of crop farmlands, and monthly application rate in hydrophytic vegetable and dry vegetable farmland growing period respectively are 4.6-13.4 times and 1.5-6.0 times those of crop farmlands whether all-year or flood and non-flood season. Each rainfall is one of the major different precipitation characteristic in the flood and Non-flood season. Each rainfall in flood season varies with that in non-flood season, each rainfall in non-flood season is 19.6mm, and that in flood season is 31.4mm. Therefore, in the study area, corresponding with that, N and P losses risk of hydrophytic vegetable and dry vegetable farmland are obviously higher than those of grain crop farmlands when precipitation occurs. The probability of nitrogen and phosphorus losses from farmlands is more in flood season than in non-flood season.Combined with the coupling between fertilizer and rainfall, seasons when non-point pollution easily happens and vegetation mulching period, the environmental effects of different crop farmlands and planting pattern farmlands in Shanghai suburb are analyzed according to nutrient balance of different crop farmlands and planting pattern farmlands over the past three years. The results show that some crops have the small risk of nitrogen and phosphorus losses, for example, long crooked squash, green soy bean, lotus root, shallot et al., but other crops have the relatively high risk of nitrogen and phosphorus losses, for example, Zizaniz Latifolia, taro, pear, leek and so on. For different planting patterns, the vegetable-vegetable rotation, the fruit tree monoculture and the vegetable-crop rotation are easier to result in the losses of nitrogen and phosphorus than the greensward only and the crop-crop rotation.