| Chemical fertilizer plays a vital role in agricultural production. Application of chemical fertilizer is the major measure to increase grain yield and agricultural income. High input of fertilizers has dramatically increased the cereal productivity and the economic benefit. However, the excessive input of chemical, especially the nitrogen(N) and phosphorus(P) fertilizers also resulted in a series of problems, which affected the environment and human health. Water scarcity and nutrient deficiency of soil are the two major limiting factors of crop growth in the Loess Plateau dryland in northwestern China. In addition, the effect of fertilizers is restricted by soil water condition, and they are a mutual influnce factors. Taking water and nutrient into consideration simultaneously in agricultural practices could avoid the waste of resources and reduce the risks to the environment. This is the research hotspot and the important improvemnent direction of soil and fertilizer science in dryland agriculture. Therefore, the main objectives of this study were to verify the responses of winter wheat biomass, grain yield, nutrient utilization, nitrate residual in soil at harvest, and nitrate leaching during summer fallow to different N and P rates in different rainfall years, and to make the scientific and reasonable fertilization scheme in combination with regional precipitation in dryland.This study was a 10-yr-long stationary field experiment which was initiated in 2004 in dryland in the central southern area of the Loess Plateau of China. There were five N(0, 80, 160, 240, and 320 kg N ha-1) and five P(0, 50, 100, 150, and 200 kg P2O5 ha-1) fertilizer rates with a randomized complete block design in four replictaions. Soil and plant samples were collected and measured during 2010-2014. Combined with the previous corresponding data and precipitation of the 10 years, we analyzed the effect of different N and P rates, and precipitation on winter wheat biomass, grain yield, nutrient utilization, nitrate residual in soil, and nitrate leaching. The main results were as follows:(1) Combined with the annual rainfall, a 10-yr long term field experiment was carried out to investigate the response of winter wheat bioamss, yield, and nutrient utilization to the N rates in different rainfall years. Results showed that winter wheat biomass, yield, N concentration in grain, and N requirement per 100 kg grain were related to the N rate quadratically and dramatically. P cncentration in grain and P requirement per 100 kg grain were significantly and negatively related to the N rate. K cncentration in grain and K requirement per 100 kg grain were related to the N rate linearly or quadratically. The partial factor productivity of N fertilizer, the accumulated apparent N recovery efficiency, and the physiological efficiency of N decreased sharply with N rate; the physiological efficiency of P increased as N rate increasing; where the physiological efficiency of K increased firstly and then decreased. With different annual rainfall, the effect of N fertilizer was different. In years with abundent rainfall, the N requirement per 100 kg grain was lower with a maximum of 3.04 kg, and in years with low precipitation it was higher with a maximum of 3.15 kg.(2) The reasonable N input for winter wheat was optimized on the basis of the safety threshold of soil nitrate residue(NR) at harvest. Obtained results noted that the NR increased quadratically with the N rates. Seasonal nitrate residue(SNR) in soil was mainly within the 0-100 cm layer with the yearly increases correspondingly being 0, 4.4, 8.8, 13.2, and 17.7 kg N ha-1 at N rates of 0, 80, 160, 240, and 320 kg N ha-1. The fertilizer N residue(FNR) increased linearly with N rate, with the yearly FNR increases, respectively, to be 0.02, 7.1, 14.1, and 21.1 kg N ha-1 at N rates of 80, 160, 240, and 320 kg N ha-1. The NR, SNR, FNR and the seasonal nitrate residual depth(NRD) were all affected by but not correlated with the precipitation. In the experimental site, it was found that the NR was able to be slowly leached downward to deeper soil by 13.3~33.3 cm each year driven by precipitation. Taking the soil nitrate residual threshold and maintaining a relatively high grain yield into consideration, the recommended N rate should be reduced to 66~92 kg N ha-1 when the grain yield was 4487~5000 kg ha-1 and the NR in the 0-100 cm soil was decreased to 55~67 kg N ha-1.(3) We selected three representative rainfall years to investigate soil nitrate leaching and accumulation during summer fallow. The objectives were to verify the major factors affecting nitrate leaching, and ascertain the relationship between the movement of soil water and nitrate. It was found that the nitrate leaching came mainly from the top 40 cm soil, and it was affected by the N rate and the summer precipitation intensity. In the wet summer of 2011, the amount of nitrate leaching increased from 14.6 to 250 kg N ha–1 as the N rate increased from 0 to 320 kg N ha–1. In the normal summer of 2012, the nitrate leaching occurred only at 240 and 320 kg N ha–1 with 47.6 and 53.8 kg N ha–1, and no leaching but accumulation occurred in the dry summer of 2013. The nitrate leahcing from top soil was found to be accumulated in deep 40-300 cm soil layers. The accumulation increased as the N rate increase, i.e., 37.7 to 387 kg N ha–1 in 2011, and 53.9 to 193 kg N ha–1 in 2012 when the N rate increased from 0 to 320 kg N ha–1. However, no accumulation was observed in deep soil in 2013. The downward movement of nitrate lagged behind that of the soil water, and 1 mm summer rainfall could cause a 1.6~3.6 mm downward movement of nitrate in the soil profile. To regulate nitrate leaching during summer fallow, the application rate of N fertilizer should not exceed 160 kg N ha–1 in this dryland area.(4) A 10-yr-long stationary field experiment was conducted to investigate the response of winter wheat bioamss, yield, and nutrient utilization to the P rates in different precipitation years. The obtained results showed that winter wheat biomass, yield, and grain N and P concentration were related to the P rate quadratically and significantly. Grain K concentration was not related to the P rate clearly. The partial factor productivity of P fertilizer decreased sharply with P rate, and the accumulated apparent P recovery efficiency was the highest at 100 kg P2O5 ha-1 where it decreased when the P rate was above 100 kg P2O5 ha-1. The physiological efficiency of N increased sharply with P rate; the physiological efficiency of P decreased as P rate increasing; where the physiological efficiency of K increased firstly and then decreased. The N and P requirements per 100 kg grain were significantly and quadratically related to the P rate, whereas the K requirement per 100 kg grain was decreased linearly as the P rate increased. The effect of P fertilizer was different in different rainfall years. In years with high rainfall, the P requirement per 100 kg grain was lower with a maximum of 0.31 kg, and in years with low precipitation it was higher with a maximum of 0.33 kg.(5) The sound application rate of P for winter wheat was optimized on the basis of the reasonable NR at harvest. Results showed that the NR decreased firstly and then increased quadratically with the P rate. The SNR in soil was mainly within the 0-100 cm layer with 82.1, 51.3, 46.6, 49.8, and 89.4 kg N ha-1, respectively, at P rates of 0, 50, 100, 150, and 200 kg P2O5 ha-1. The NR in the 0-300 cm and 0-100 cm soil layers, and the SNR increased as time passing and were described by binary quadratic equations. It showed that optimizing the rate of P fertilizer could regulate soil nitrate residue effectively. Soil nitrate residue could be decreased, and the grain yield was relatively high with 5500~5741 kg ha-1 at a reasonable P application rate of 104~168 kg P2O5 ha-1.(6) Based on three representative rainfall years, we analyzed the soil nitrate leaching and accumulation during summer fallow and verified the effect of P rate on nitrate leaching. The results showed that the nitrate leaching occurred mainly in the top 40 cm soil, and it was affected by the P rate and the summer rainfall intensity. In the wet summer of 2011, compared with the other P rates, only the P rate of 100 kg P2O5 ha–1 significantly decreased the amount of nitrate leaching which was 88.2 kg N ha-1. In the normal and dry summers of 2012 and 2013, no leaching occurred. The nitrate lost from top soil was noted to be accumulated in deep 40-300 cm soil layers, and the application of P fertilizer could decrease its amount. The accumulation decreased from 196 to 134 kg N ha–1 in 2011, and from 134 to 55.9 kg N ha–1 in 2012 as the P rate increased from 100 to 200 kg P2O5 ha–1. However, no accumulation was observed in deep soil in 2013. In order to prevent nitrate leaching and reduce its accumulation in deep soil, the application rate of P fertilizer should not lower than 100 kg P2O5 ha–1 in this area.In general, it is necessary to make reasonable N and P fertilizer rates on the basis of crop nutrient requirement in different rainfall years. Regulating soil nitrate residue at harvest and leaching during summer fallow is of important significance in agricultural production in dryland of the Loess Plateau in China. |
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