Effect Of Cultivation And Tillage Measures As Well As Long-term Nitrogen Fertilization On Crop Yield And Soil Fertility Quality In Dryland

Food security remains a major challenge facing the world today. Wheat is one of the world’s four most important food crops, and more than 75% of this production occurs in dryland regions, including irrigated dryland. The major limitations of these regions are water shortages and nutrient deficiencies. There is a large gap between actual and potential productivity for wheat grown in dry climates, due to water limitation. Therefore, sustainable water and nutrient management practices are important to ensure a sufficient food supply for the growing population in such regions. N application significantly increases wheat yield on infertile soil. However, long-term unreasonable application of N usually results in serious environmental degradation. There were some researches about water and N use efficiency under different soil surface mulching practices, but little on the assessment and analysis of soil quality in this dryland system. Additionally, the difference of contribution of inorganic and organic fertilizer on wheat yield and soil fertility was not clear yet. Therefore, to evaluate the sustainability of dryland system plays an important role in food production security and sustainable agriculture. The dissertation studies three long-term field trials and farmer’s soil samples from different locations in Guanzhong Plain of Shaanxi province. There are three combinations consisted of this study, including soil surface mulching coupled with N fertilizer application, organic coupled with inorganic fertilizer, and tillage coupled with residue management. The main conclusion showed as follows:(1) The experiment was conducted during the seven consecutive years of 2002-2009 in Guanzhong Plain of Shaanxi province. The effects of surface mulching and N addition on winter wheat yield and soil nutrients regimes were compared. The nine treatments consisted of three levels of mulching, namely, no mulching(NM), wheat straw mulching(SM), ridge mulched with plastic film + bare furrow(RM), and three nitrogen levels(N 0, 120 and 240 kg/ha). Soil samples were collected after wheat harvest in 2009 of a winter wheat-summer fallow system. In addition, wheat yields were also considered in the growing seasons of 2002-2009. The results showed that the yield was averagely decreased by 1.46% in the SM treatment, and increased by 7.04% in the RM treatment compared to the NM treatment of consecutive seven years. Moreover, the yield was averagely increased by 42.72% in the two nitrogen amended treatments compared to the no nitrogen addition treatment during 2002-2009. However, no significant differences between N application of 120 and 240 kg/ha treatments were found. Comparing with NM treatment, soil labile organic C concentration at the surface layer(0-20 cm) was significantly increased by 37.57% and 21.39% in the SM and RM treatments, respectively. In contrast with RM, a significant increase of 6.84%, 4.40% and 12.68% in soil organic C, total N and available K was observed in SM treatment, respectively. Soil labile organic C and mineral N were averagely increased by 6.03%, 40.18%, and available P and p H value were decreased by 23.55% and 0.05 p H unit in the two N amended treatments compared to the N0 treatment.(2) The experiment was conducted during the night consecutive years of 2002-2010. To evaluate the sustainability of dryland system under soil surface mulching and N application management, sustainability index was calculated. The results showed that the effect of N application rate on soil available P, available K was different with the effect on soil organic C, total N and alkali-hydrolysable N, while the same effect found under mulching management practices. The largest nutrient index and microbial index was found under SM. But there were no significant differences of nutrient index and microbial index between NM and RM. Crop index was more responsive to N application rate. The sustainability index was higher under SM, followed by those for NM and RM, and higher under N120, followed by N240 and N0. Without N application, sustainability index was lowest, while SM + N120 resulted in higher sustainability. RM coupled with N application did not produce high sustainability index.(3) The experiment was conducted during the nine consecutive years of 2002-2011. The effects of soil water, organic C, and N management practices on winter wheat yield and soil nutrients regimes were compared, using treatments that included nine factorial combinations of three cultivation practices, conventional cultivation(CC), straw mulching(SM), and supplementary irrigation(SI), and three N application rates(N 0, 120, and 240 kg/ha). Relative yield gradually declined under CC and SM with N, yet remained steady under SI. Without N, yield decreased by 50 to 60%. Soil organic C, labile organic C, total N, and available K in the 0 to 20 cm(upper) soil layer were significantly increased by SM but were unaffected by SI treatments. After wheat harvest, N application increased organic C, labile organic C, and total N, in the upper soil layer by an average of 4.81, 20.70, and 7.61%, respectively, and decreased available K by 6.12%. The cultivation practice and N fertilizer effects on soil properties were more pronounced in upper than deeper layer(20-40 cm). At soil depths of 0 to 100 cm, nitrate accumulation under N240 exceeded 69.27% of the critical environmental risk value.(4) The experiment was conducted during the ten consecutive years of 2002-2012. The effects of straw and N application on winter wheat yield and soil nutrients regimes were compared. The results showed that the wheat yield responded more to the N application rate than to cultivation practices. The reduction of wheat yield under straw application, increment under higher N application, and reduction under straw coupled with N application were decreased with N application rate. Additionally, higher N application narrowed the difference in the main effect of straw(S) and N on soil organic C, labile organic C, total N, and available K between 5-15 cm and 15-30 cm soil layer. Higher N application showed more positive effect on soil fertility in the lower soil depth. The N main effect of soil available P was higher than the S main effect, whereas the response of available K was opposite regardless of N application rate, indicating that straw application did not offset the reduction of soil available P, but could offset available K losses.(5) To study soil nutrient distribution in the 0-30 cm soil depth under straw mulching in the dryland system, three sites(Weinan, Yangling, Sanyuan) were selected. The results showed that SM significantly increased soil organic C, labile organic C, and total N in the 0-15 cm soil depth at Weinan, and in the 0-5 cm depth at Sanyuan. Compared with no mulching, the increment of soil C and N in the 0-5 cm soil depth under SM at Weinan was higher than that of in the 5-15 cm depth, while the decrement of soil C and N in the 15-30 cm soil depth under SM at Sanyuan was higher than that of in the 5-15 cm depth. SM produced higher fertility in the 0-30 cm depth at Yangling. SM increased soil organic C, labile organic C, available P, and available K in the 0-5 cm soil layer by an average of 12.43%、20.30%、16.21%、23.73%。The relative yield responded more to the N application rate than to cultivation practices. Soil organic C did not response to SM as rapidly as labile organic C did in the 0-15 cm depth at three sites. Factor analysis showed that there was some difference in soil C and N between the 0-15 cm depth and the 15-30 cm depth. SM produced higher stratification of soil properties. The stratification ratio(0-5 cm: 15-30 cm) of soil organic C exceeded the critical value(SR = 2) under SM at Weinan and Sanyuan. Additionally, the stratification ratio of available nutrients under SM was higher than that of total C and N, especially higher ratio was obtained under available P at Weinan(5.24) and Sanyuan(7.93).(6) The experiment was conducted during the seven consecutive years of 2002-2009. The effects of tillage and residue management on soil properties were compared in a winter wheat-summer maize cropping system. Treatments included 8 factorial combinations of four tillage practices: subsoiling tillage(ST), rotary tillage(RT), no tillage(NT), conventional tillage(CT), and maize residue(with or without) management. Soil samples were collected after wheat harvest in June 2009 and maize harvest in October 2009. The highest soil bulk density in the 0-10 cm soil layer after maize harvest was observed under NT, followed by that for CT and RT, and the lowest value was observed under ST. Compared with maize residue removed treatment(-r), soil bulk density in the 0-10 cm soil layer under residue retained treatment(+r) significantly decreased by 2.33%. Multivariate analyses were used to select appropriate soil quality indicators. The results showed that the three soil property groups(i.e. soil nutrient group, soil organic matter and environment group, enzymatic activity group) responded more to tillage practice than to residue management. CT resulted in the poorest soil quality in both seasons. The residual effects of ST and RT on soil quality were found in maize season. Additionally, scores for soil organic matter and nutrient factor under ST and RT were higher in both seasons, but the opposite occurred in NT.In general, SM, RM, SI coupled with N fertilizer did not meet both high soil fertility and high productivity needs in this region. SM + N120 increased soil fertility, SI + N120 achieved a high and stable wheat yield, while RM coupled with N was not sustainable in the studied site. Applied higher N, would produce more wheat yield, and increase soil fertility in the lower soil depth, but worse environment effect. ST and RT coupled with SM produced higher soil quality, and decreased stratification of soil properties. Hence, additional treatment groups of SM + SI combined with moderate N application or SM + ST + RT would meet both high soil fertility and high productivity needs.