4.1 High surplus and low NUE at intensive orchard region
Our study showed that nutrient surpluses in the soil of both two counties have increased significantly since 1992 (Fig. 2) due to the rapidly increasing application of synthetic fertilizers in China since the 1980s (Bellarby et al., 2017; Sun et al., 2020; Yu et al., 2020). N and P surpluses of the two counties are higher than that of other studies. For example, the annual average N surplus (272 kg N ha-1 in Wugong) was much higher than that of cereal lands in the Sanjiang Plain (22.2 kg N ha-1) and Haihe basin (159 kg N ha-1) (Zheng et al., 2015; Yang et al., 2018). The nutrient surpluses in Meixian County were significantly higher than that of Wugong County which is consistent with other studies (Sun et al., 2008). This suggests that both the rapidly increasing application of synthetic fertilizers and LUC from cereal to intensively horticultural crops contributed to the high nutrient surpluses in soils.
It is expected that the high nutrient surpluses resulted in a significant decrease in NUEs from 1992 to 2017 in two counties (Fig. 3), which is consistent with previous studies (Ma et al., 2012). NUEs of Meixian (19-31%) was significantly lower than that of Wugong (30-54%), which could be explained by the high nutrient surpluses at Meixian (Fig. 3). The reasons for high nutrient surpluses and low NUEs in the intensive horticultural crop regions are as follows: (i) The incomes of intensively horticultural crops are usually much higher than that of cereals. The smallholder farmers are usual with poor knowledge of better fertilizer management. They tend to overfertilize as an “insurance” to avoid yield losses (Yin et al., 2021). (ii) Unlike in Europe and North America, there are still no regulations or laws in most regions of China to limit nutrient surpluses in the soil to protect the environment (Hou et al., 2021; Xin et al., 2021). It is also the situation in other developing economies (Morseletto, 2019). Therefore, more urgent attention should be paid to the issues of high nutrient surpluses and low NUEs in intensively horticultural crop regions.