Ammonia oxidization is a critical process in nitrogen cycling that involves ammonia oxidizing archaea (AOA) and bacteria (AOB). However, the effects of different manure amounts on ammonia-oxidizing microorganisms (AOMs) over the course of organic vegetables production remains unclear. We used the amoA gene to evaluated AOMs abundance and community structure in organic vegetable fields. Quantitative PCR revealed that AOB were more abundant than AOA. Among them, the amoA copy number of AOB treated with 900 kgN ha⁻¹ was 21.3 times that of AOA. The potential nitrification rate was significantly correlated with AOB abundance (P < 0.0001) but not with AOA, suggesting that AOB might contribute more to nitrification than AOA. AOB sequences were classified into Nitrosomonas and Nitrosospira, and AOA into Nitrosopumilus and Nitrososphaera. Nitrosomonas and Nitrosopumilus were predominant in treatments that received manure nitrogen at ≥ 900 kg ha⁻¹ (52.7–56.5%) and when manure was added (72.7–99.8%), respectively, whereas Nitrosospira and Nitrososphaera occupied more than a half percentage in those that received ≤ 600 kg ha⁻¹ (58.4–84.9%) and no manure (59.6%). A similar manure rate resulted in more identical AOMs’ community structures than greater difference manure rate. The bacterial amoA gene abundances and ratios of AOB and AOA showed significantly positive correlations with soil electrical conductivity, total carbon and nitrogen, nitrate, phosphorus, potassium, and organic carbon, indicating that these were potential key factors influencing AOMs. This study explored the AOMs’ variation in organic vegetable fields in Northwest China and provided a theoretical basis and reference for the subsequent formulation of proper manure management.

Accurate estimation of crop evapotranspiration (ET_c) is important to determine crop water requirements in greenhouse agriculture and to improve the irrigation water use efficiency. Here, a 3-year (2017–2019) experiment with spring greenhouse-grown eggplant (Solanum melongena L.) was conducted to investigate variation in the crop coefficient (K_c) measured with a weighing lysimeter, adjust K_c based on the local climate (K_c,Adj), and estimate daily ET_c using a crop coefficient model. The 3-years mean local K_c (K_c,Loc) were 0.23 ± 0.03, 0.62 ± 0.06, 1.05 ± 0.03 and 0.87 ± 0.03 at the initial, development, mid-season, and end-season stages, respectively. Significant linear correlation was observed between K_c,Adj and K_c,Loc in the 3 years (R² = 0.873, 0.901, and 0.897 in 2017–2019, respectively). Compared with the FAO-56 recommended K_c value (K_c,FAO), the mean K_c,Adj and K_c,Loc in the 3 years were by 66.3% and 61.8% lower, respectively. The single crop coefficient model accurately estimated daily ET_c for greenhouse-grown eggplant. The coefficient of determination (R²), mean absolute error (MAE), root-mean-squared error (RMSE), and index of agreement between measured ET_c and that estimated by the single crop coefficient model were 0.94, 0.35 mm‧d⁻¹, 0.26 mm‧d⁻¹, and 0.98, respectively, for the means in 2018 and 2019. Therefore, the crop coefficient method reliably estimated evapotranspiration with adjustment for the actual environment and can serve as a useful tool to improve water use efficiency.

The application of large amounts of manure nitrogen (N) in Northwest China has had a serious negative impact on the sustainable development of regional organic agriculture and ecological environmental protection. Field experiments were conducted in three crop cycles in Ningxia, Northwest China, to study the effects of different manure application rates on the N absorption and utilization of Chinese Flowering Cabbage (CFC) and N distribution characteristics of manure, which were carried out on the basis of 0(M₀), 300(M₃₀₀), 600(M₆₀₀), 900(M₉₀₀), 1200(M₁₂₀₀) kg N·hm^-2 manure N rates by setting an ¹⁵N micro-area. Results showed that the ¹⁵N absorption in each crop of CFC showed a parabola trend of "low high low" with an increase in the amount of manure, and the ¹⁵N absorption of the M₉₀₀ treatment was the highest, which increased by 64.3 % compared with that of the M₁₂₀₀ treatment (P<0.05). After three crops of CFC, only 10.2–24.0% of manure N was absorbed by the crops, and 39.0–54.3% remained in the 0–100 cm soil layer, with a loss ratio of 35.5–48.5%. The manure N absorption rate under M₉₀₀ treatment reached the maximum (24.0 %), and the amount of soil residual (351.11 kg·hm^-2) was greater than the loss (333.28 kg·hm^-2). N absorption in the M₁₂₀₀ treatment was 39.2 % lower than that in M₉₀₀ (P<0.05), and the loss (581.17 kg·hm^-2) was greater than that of the residue (487.64 kg·hm^-2). The residual ¹⁵N was mainly distributed in the 0–40 cm soil layer, and the residual amount was 127.65–390.32 kg·hm^-2, which accounted for 78.1–81.3% of the total residual amount. The nitrate-N content in the 0–20 cm and 20–40 cm soil layers increased with increasing manure application rates, and there was no significant difference between the M₉₀₀ and M₁₂₀₀ treatments. However, the NO₃^–-¹⁵N content in the 40–100 cm soil layer in the M₁₂₀₀ treatment was 1.2 times higher than that in the M₉₀₀ treatment (P<0.05). Therefore, considering the absorption, residue, and loss of manure N in crops, it can be concluded that a manure N application rate of 900 kg N·hm^-2 is the most favorable for the production of three consecutive crops of CFC.