Spatiotemporal variations in planting structure and nitrogen surplus in the Baiyangdian basin

Abstract: Environmental loss can be caused by the excessive nitrogen input in crop production, leading to non-point source pollution in the lake basin. Among them, the planting structure can be one of the most important factors to affect the nitrogen input, due to the various levels of nitrogen management between different systems of crop production. Therefore, it is very necessary to explore the relationship between planting structure and environmental loss at present. In this study, the grain crops (wheat and maize) and economic crops (vegetables and fruit trees) were taken as research objects in Baoding and Dingzhou from the core areas of the Baiyangdian Basin in the North China Plain. An optimization was made to clarify the effect of planting structure on the regional environment, where the grain economy ratio was taken as the characterization factor of planting structure, while the nitrogen surplus was the characterization factor of environmental loss. Specifically, the grain economic ratio was the planting area ratio of grain crop to economic crop. Nitrogen surplus was the difference between nitrogen input and output in the planting system. The results showed that: 1) There were outstanding spatiotemporal changes in the planting structure in the Baiyangdian Basin. In the spatial view, the largest grain economic ratio was found nearby the Baiyangdian Lake area, whereas, the smallest grain economic ratio was upstream of the Baiyangdian Lake. The grain economy ratio decreased significantly with the increase in distance from the spatial divisions to the lake. In terms of time, the grain economic ratio and each spatial partition showed a slight upward trend during the study period (in 2013, 2015, 2017 and 2019) among the four spatial divisions. There was the largest increase in the grain economy ratio in the middle reaches of Baiyangdian Lake; 2) The spatial and temporal differentiation characteristics of planting nitrogen surplus in Baiyangdian Basin were obvious. In terms of time, the planting nitrogen surplus in Baiyangdian Basin showed a downward trend ( 26.29×104-18.49×104 t ) during the study period. Spatially, the nitrogen surplus in the midstream and downstream of the basin is larger than those in the upstream and nearby Baiyangdian Lake. 3) There was no change in the total planting area of grain crops and economic crops in the four spatial zones, but the nitrogen surplus decreased significantly. Among them, the most decrease in the nitrogen surplus was found in the midstream of Baiyangdian. The trend was attributed to the small basic value of the grain economic ratio and the large increase in the grain economic ratio. Furthermore, the nitrogen surplus decreased with the increase in the grain economy ratio, while the planting area remained unchanged basically in the same area. In addition, the ever-increasing trend was observed in the grain economy ratio with the decrease of the basal value, leading to the decrease of nitrogen surplus; 4) There was a strong relationship between the grain economic ratio and nitrogen surplus intensity. The smaller the grain economic ratio was, the greater the nitrogen surplus intensity was. The nitrogen surplus intensity also decreased with the increase in the grain economic ratio. Consequently, the regional nitrogen surplus intensity can be expected to be significantly adjusted via the grain economic ratio. The nitrogen surplus intensity in the midstream of Baiyangdian Lake could be reduced to the level of II (140-180 kg/hm2) from the level of IV (220-260 kg/hm2), when the grain economy ratio is adjusted from 4.21 to 12.13-17.79. Therefore, the nitrogen surplus intensity can be regulated to adjust the planting structure, in order to control the nitrogen surplus for the less environmental risk of nitrogen input in planting industry. This finding can provide a scientific basis for the control of nitrogen pollution in the planting industry.