Rice intercropping with water mimosa (Neptunia oleracea Lour.) could alleviate the negative effects of simulated nitrogen deposition on rice and soil

Nitrogen (N) deposition usually has adverse effects on various ecosystems. Farmland intercropping is a well-known planting model and agroecosystem with multiple benefits. However, research on N deposition in farmland, especially for intercropping systems remains limited.

Field experiments were conducted in three seasons to examine the effects of rice monocropping and rice-water mimosa (Neptunia oleracea Lour.) intercropping systems on rice growth and soil properties under N deposition. The simulated N deposition rate was set at two levels, which included low N (LN) deposition at a rate of 40 kg·ha⁻¹·yr⁻¹ N and high N (HN) deposition at a rate of 120 kg·ha⁻¹·yr⁻¹ N, which were applied in addition to 180 kg·ha⁻¹·yr⁻¹ N fertilization during the overall growth period of rice.

With increasing N deposition, rice plant height, average root diameter and water mimosa yield, above-ground dry weight decreased in the monocropping treatment. Meanwhile, water mimosa yield, above-ground dry weight, and all of the indices of root morphology in 2022 early season decreased with the increasing N deposition in the rice-water mimosa intercropping system. Contents of soil total manganese (Mn), total zinc (Zn), total calcium (Ca), and cellobiosidase activity also declined with increasing N deposition. However, the land equivalent ratio (LER) of the intercropping system was greater than 1 even under N deposition. In addition, compared with monocropping, intercropping increased dry weight of stem and leaves, average root diameter of rice, contents of soil total nitrogen, total phosphorous, total Mn, total Zn, total Ca and the activity of acid phosphatase, and also enhanced soil microbial biomass carbon (MBC), nitrogen (MBN) and phosphorus (MBP), gram-positive bacteria, gram-negative bacteria, fungi, bacteria, methane-oxidizing bacteria contents and fungi/bacteria ratio.

The experiment results suggest that N deposition caused negative impacts on rice farming system. However, rice and water mimosa intercropping systems can reduce the negative effects of N deposition (especially LN) on rice and soil. The findings demonstrate that this intercropping system is advantageous under N deposition (especially LN) than rice monocropping.