Response of microbial necromass nitrogen to nitrogen fertilization rates in a potato continuous cropping soil in North China

Objectives Microbial residual nitrogen is a stable and important soil nitrogen (N) component that plays an important role in maintaining soil fertility. The amount of N fertilizer applied affects the changes and accumulation of soil microbial residual N. Understanding the response of microbial residual N to different N application rates is crucial for optimizing N management to improve soil fertility.

Methods The potato N fertilizer management experiment was conducted in 2017 at the Wuchuan Dry Crop Experimental Station in the Inner Mongolia Autonomous Region, using a potato continuous cropping system with four treatments: NE fertilization recommended using the NE (Nutrient Expert) system, NE−N (no N fertilizer), NE−1/2N (N fertilization rate decreased 50% than NE), and NE+1/2N (N fertilization rate increased 50% than NE). Soil samples from the 0−20 and 20−40 cm soil layers were collected after potato harvest in 2023. The contents of soil mineral N and microbial necromass N were determined, and the effect of different N fertilizer rates on soil mineral N and microbial necromass N under the continuous potato cropping system was evaluated.

Results N addition increased soil total N (TN) and NO₃⁻-N compared to NE−N. Soil NO₃⁻-N and NH₄⁺-N increased with increasing N rates in the 20−40 cm layer, but soil TN, NO₃⁻-N, and NH₄⁺-N in the 0−20 cm depth did not differ across these three N fertilizer treatments. Compared with NE, NE−N and NE−1/2N decreased soil bacterial, fungal, and microbial necromass N, whereas NE+1/2N did not increase these necromass N. Across all treatments, the ratio of fungal necromass N to bacterial necromass N gradually decreased with increasing N fertilizer rates, and the ratio in the 20−40 cm layer was higher relative to that in the 0−20 cm layer. The proportion of microbial necromass N to soil TN reached 64.2%−73.2% across all treatments. N rates did not affect the proportion in the 0−20 cm depth; however, NE−N, NE−1/2N, and NE+1/2N decreased the proportion of microbial necromass N by 4.1%−13.2% compare to NE in the 20−40 cm layer. The NE treatment maintained a good soil N balance, while the NE+1/2N treatment resulted in a significant soil N surplus.

Conclusions In the potato continuous cropping system of Inner Mongolia, soil microbial necromass N gradually increased with increasing N rates, and the N fertilization recommended using the NE system achieved higher microbial necromass N accumulation. Therefore, the application of fertilizer based on the NE system can maintain soil fertility and field N balance.