Long-Term Fertilization Effect on Crop Yield and Water Use Efficiency in a Rainfed Cropland on the Loess Plateau.

The types of landform in the Loess Plateau of China include tableland, ridge, hill and gully, etc. Water shortage is one of the most important characters in this region. The Changwu tableland is located in the southern Loess Plateau and has sub-humid climate subjected to drought. The rainfed farming is the main agricultural production practice. In 1984, a long-term rotation and fertilization experiment was set up in the Changwu tableland, in which there were 36 treatments with 108 plots and 66.7 m2 of area for each plot. Fertilization systems include N application only, P application only, organic manure (M) only, and mixed applications of N, P and M. The dominant crops are wheat (Triticum aestivum L.) and maize (Zea mays L.). The experiment has generated a series of academic publications with many significant accomplishments over the years.

Currently in the Southern Loess Plateau, important production issues include evaluating crop yield and water use efficiency in different fertilization treatments in rainfed agricultural ecosystem and understanding of the mechanism of soil water use under high yielding and high fertilizer inputs. Under rainfed conditions, fertilization can significantly promote crop growth, and increase crop yield and water use efficiency. Meanwhile, it will also increase water consumption and aggravate soil water deficit. In this paper, the research progress of rainfed wheat yield and water use efficiency under long-term fertilization is reviewed.

(1) By comparing the inter-annual variability of crop production in different fertilization treatments, we verified the important role of mixed N and P, mixed organic and inorganic fertilizer for maintaining the high crop yield and improving water use efficiency. The use efficiency of N and P fertilizer fluctuated with annual precipitation, and the variation of N use efficiency and P use efficiency among years are 6.4%-61.0% and 3.7%-36.8%, respectively. Based on the long-term experiment, the optimized mode of fertilizer application in different precipitation years are established, which suggested increasing N fertilizer in order to increase crop yield in wet years(N:P=3:1) and increasing P fertilizer to stabilize crop yield(N:P=1:1) in dry years, and maintaining N:P=2:1 in normal years.

(2) The average yield and precipitation use efficiency (PUE) of fertilized wheat field are 1-2 times higher than those of control. For over 20 years, with the fertilization rate of 90-180 kg N ha-1, wheat yield increased 38-49 kg ha^-1a^-1 (p<0.05) and PUE increased 0.071-0.088 kg ha^-1a^-1. As N application increased, crop yield increased correspondingly. Also, the impact of annual precipitation on crop yield appeared more significant, particularly the precipitation during fallow season.

(3) After more than 20 years of fertilization, the rainwater infiltration depth in N, mixed NP and mixed N, P, M treatments decreased 10cm, 30cm, and 60cm, respectively, compared to the CK. At the same time, precipitation storage efficiency (PSE) during the fallow season in N, mixed NP and mixed N, P, M treatments increased from 28% to 32%, 34% and 33%, respectively, compared to the CK. These results indicate that, fertilization resulted in more soil water consumption during crop growth periods. However, fertilization also increased soil organic carbon, which can improve the soil water-holding capacity, thereby increasing the precipitation storage efficiency and water supply during the fallow season.

(4) In the tableland of the Loess Plateau, no soil dry layer was found in rainfed farmland during the years with medium and low yields. With the increase of fertilizer input, dryland crop yield increased correspondingly but the main limiting factor changed from nutrient factor to soil water, and soil dry layer appeared in high crop yield farmland. However, those dry layers decreased or disappeared in the years of abundant rainfall (once every 10 years), which has not become the long-term obstacle for the sustainable development of regional food production.