Optimizing the synthetic nitrogen rate to balance residual nitrate and crop yield in a leguminous green-manured wheat cropping system

Nitrogen management indicators for sustainable crop production in an intensive potato system under drip irrigation

Reliable nitrogen (N) fertilizer management indicators are essential for improving crop yields and minimizing environmental impacts for sustainable production. The objectives of this study were to assess the importance of major N management indicators (NMIs) for higher yield with low risks of environmental pollution in an intensive potato system under drip irrigation. Six drip-irrigated field experiments with no N application (Control), farmer practice (FP), and optimized N management (OM) based on N-balance, soil mineral N (Nmin), and target yield were conducted from 2018 to 2020 in Inner Mongolia, China. The response of NMIs to potato yield and yield-based environment impact indices (EIY) was evaluated by the random forest algorithm. The N input, N losses from N leaching, ammonia (NH3) volatilization, nitrous oxide (N2O) emission, N use efficiency (NUE), N surplus, and soil residual N after harvest were obtained to identify the best NMIs for high yield and minimal ecological impact. The N management practices in field experimental sites affected the importance of the order of NMIs on potato yield and EIY. The NUE and N leaching were identified as the highest importance scores and the most essential controlling variables to potato yield and EIY, respectively. The integrated NUE and N leaching indicator played a vital role in improving potato yield and reducing ecological impact. The OM treatment achieved 46.0%, 63.6%, and 64.6% lower in N application rate, N surplus, and reactive N loss, and 62.4% higher in NUE than the FP treatment while achieving equal potato yields, respectively. Those key NMIs can guide farmers in understanding their practice short comes to achieve both high productivity and environmental sustainability in intensive potato production systems under drip irrigation.

Combined Application of Leguminous Green Manure and Straw Determined Grain Yield and Nutrient Use Efficiency in Wheat-Maize-Sunflower Rotations System in Northwest China

Leguminous green manure (LGM) has a reputation for improving crop productivity. However, little is known about the beneficial interactions with straw on crop yield and nutrient (N, P, K) use efficiency. Herein, a 9-year field experiment (from 2015 to 2023) containing three treatments-(1) chemical fertilizer as the control (CK), (2) NPK + straw return (Straw) and (3) NPK + straw return with LGM (Straw + LGM)-was conducted to investigate whether the combined application of LGM and straw can increase productivity and nutrient use efficiency in the wheat-maize-sunflower diversified cropping rotation. The results showed that in the third rotation (2021-2023), Straw + LGM significantly increased wheat yield by 10.2% and maize yield by 19.9% compared to CK. The total equivalent yield under Straw + LGM was the highest (26.09 Mg ha-1), exceeding Straw and CK treatments by 2.7% and 12.3%, respectively. For each 2 Mg ha-1 increase in straw returned to the field, sunflower yield increased by 0.2 Mg ha-1, whereas for each 1 Mg ha-1 increase in LGM yield from the previous crop, sunflower yield increased by 0.45 Mg ha-1. Compared to CK, the co-application of LGM and straw increased the N use efficiency of maize in the first and third rotation cycle by 70.6% and 55.8%, respectively, and the P use efficiency by 147.8% in the third rotation cycle. Moreover, Straw treatment led to an increase of net income from wheat and sunflower by 14.5% and 44.6%, while Straw + LGM increased the net income from maize by 15.8% in the third rotation cycle. Combining leguminous green manure with a diversified cropping rotation has greater potential to improve nutrient use efficiency, crop productivity and net income, which can be recommended as a sustainable agronomic practice in the Hetao District, Northwest China.

Long-term maintenance of high yield and soil fertility with integrated soil-crop system management on the Loess Plateau

Ridge-furrow with full film mulching has been widely applied to increase crop yield and water productivity on the Loess Plateau, but it may stimulate carbon (C) mineralization. How to integrate other technological benefits based on this technology for long-term maintenance of high yield and soil fertility is a pressing issue. With the local farmers' practice (FP) as a control, three integrated soil-crop system management (ISSM) practices integrating fertilizer rates, fertilizer types and planting densities (ISSM-N1, ISSM-N2 and ISSM-MN) were established to improve maize yield and soil quality. Compared with the FP, the maize yield increased by 13.34%, 21.83% and 30.24%, and the soil quality index (SQI) increased by 9.66%, 14.91% and 38.38% for ISSM-N1, ISSM-N2 and ISSM-MN, respectively. However, ISSM-N1 did not significantly increase yield, and ISSM-N2 increased residual soil nitrate and decreased nitrogen (N) partial factor productivity significantly. Compared to the FP, ISSM practices increased soil organic carbon (SOC), labile organic C fractions (LOCFs) and potassium permanganate organic C fractions in the topsoil to varying degrees, but only ISSM-MN reached significant levels for most C fractions. The sensitivity index indicated very easily oxidizable C (24.6%), easily oxidizable C (24.7%), hot-water extractable C (30.8%), labile organic C (24.7%) and particulate organic C (57.3%) were more sensitive than SOC (22.7%). ISSM-MN sequestered significantly higher C than the other treatments. The results of the relative importance analysis and the structural equation model indicated that LOCFs were the direct contributors to yield, while recalcitrant C (CO) was the indirect contributor, revealing the underlying mechanism that CO decomposed to replenish LOCFs and the total N pool with the water soluble C pool as the transit station. Overall, ISSM-MN is the most promising strategy to improve crop yield and soil fertility in the long term on the Loess Plateau.

Adaptive evaluation for agricultural sustainability of different fertilizer management options for a green manure-maize rotation system: Impacts on crop yield, soil biochemical properties and organic carbon fractions

Green manure planting can reduce the intensity of soil use, while improving farmland productivity in double-cropping systems. However, only few studies have focused on the impacts of green manure application under different fertilization management options on succeeding crop yield and soil organic carbon (SOC) process. A three-year field experiment was conducted with a winter smooth vetch-summer maize cropping system to evaluate the effects of green manure with different chemical fertilizers on soil physiochemical properties, SOC fraction, enzyme activities and maize yield. Total eight treatments were compared including different combinations of green manure and chemical fertilizers (i.e., nitrogen and phosphorus fertilizers) in the smooth vetch phase and maize phase. The results showed that compared to the control, green manure incorporation increased the soil moisture, total nitrogen, total phosphorus, basal respiration, SOC and its labile fractions, and enzyme activities, especially for the treatments of green manure with fertilization. However, the soil pH and bulk density decreased due to green manure application. Maize yield increased 34 %-53 % after green manure application, and was found to be significantly and positively correlated with soil carbon process (P < 0.05). Moreover, SOC and its labile fractions, and total nitrogen were observed as the main drivers of the maize yield. Variation partition analysis demonstrated that soil biochemical properties and their interaction with green manure by fertilization caused variations in SOC fractions. Further, structural equation models indicated that both balanced fertilization practices had positive effects on maize yield and soil carbon process via changes in SOC fractions and C cycling-related enzyme activities, respectively. In addition, the amount balance of chemical fertilizer positively impacted the soil carbon process by regulating SOC fractions through enzyme activities. These findings provide important guidance for applying optimal fertilization management in the green manure phase to improve succeeding crop yield and soil quality as well as to mitigate the adverse impacts of chemical fertilizers. The study will be equally illuminating for other green manure-crop rotation systems.

Evaluating the significance of amino acids (AAs) in cyanide-treated rice plants under different nitrogen fertilization using the relative importance index of AA

The biosynthesis of amino acids (AAs) in plants is affected by different nitrogen (N) sources. The effects of exogenous cyanide (KCN) on the concentrations and profiles of AAs in rice seedlings were carried out in the presence of nitrate (+NO₃^-)/ammonium (+NH₄⁺) or N deficiency (-N). Targeted metabolomics analysis indicated that the highest accumulation of AAs in CN^--treated rice seedlings was detected in the "CN^-+NH₄⁺" treatments than in other treatments, wherein the doses of exogenous KCN did not significantly affect the total amount of AAs in rice seedlings at the same N fertilized condition. The total content of AAs in rice shoots under "CN^-+NH₄⁺" treatments was higher than other treatments, while the total content of AAs in rice roots under "CN^-+NO₃^-" treatments was higher than other treatments. Also, the profiles of 21 AAs in CN^--treated rice seedlings showed tissue-specific under different N fertilization. The relative importance index (RII) of AA was used to evaluate the importance of AAs in CN^--treated rice seedlings under different N fertilization. The common AAs with higher RII values were compared between three different treatments of KCN (e.g., 0, 1, and 2 mg CN/L). Under "CN^-+(-N)" treatments, Ala, Asp, Glu, Val, and Gly (Ala, Gly, Val, and Lys) were the common AAs in rice roots (shoots). Under "CN^-+NO₃^-" treatments, Ala, Glu, Asp, Ser, and Thr (Asp, Ala, Thr, Ser, and Asn) were the common AAs with higher RII values in rice roots (shoots) between all CN^- treatments. Under "CN^-+NH₄⁺" treatments, Asp, Gln, Asn, and Ala (Asp, Glu, and Thr) were the common AAs with higher RII values in rice roots (shoots) between all CN^- treatments. These results suggested that using the RII to describe the change and fluctuation of AAs in rice plants may reflect the different N utilization strategies in response to exogenous CN^- exposure.

Soil surface management of legume cover has the potential to mitigate nitrous oxide emissions from the fallow season during wheat production

Soil surface management, i.e., mulch by film, straw or cover crop, is very important to water availability in soil on drylands worldwide, especially during the fallow season, when there is a high concentration of soil nitrate nitrogen (N) to produce nitrous oxide (N₂O). To determine whether soil surface management affects N₂O emissions during the fallow season, we conducted an experiment to compare N₂O emissions from a wheat field that received different surface soil management strategies: control (CK), straw mulch and incorporation (SR), planting legume green manure and incorporation (GM), SR plus GM (SR + GM), and plastic film mulch (FM). The results showed that the average hourly N₂O emissions during the fallow season were in the order SR (7.4 μg N m^-2 h^-1), GM (10.7 μg N m^-2 h^-1), SR + GM (11.7 μg N m^-2 h^-1), FM (15.5 μg N m^-2 h^-1), and CK (16.4 μg N m^-2 h^-1). Correspondingly, reduced total N₂O emissions were observed in the SR, GM and SR + GM treatments, with an average reduction of 39.0% (from 302 to 184 g N ha^-1) while increased N₂O emissions were from the GM and SR + GM treatments in the wheat growing season. Additionally, N₂O emissions were related to soil nitrate N content, microbial biomass and moisture. Overall, considering N₂O emissions, C and N inputs by plant residues and grain yield, the management of GM has the potential to reduce greenhouse gas emissions and improve soil C sequestration and soil fertility. These results emphasized the importance of legume green manure to wheat-fallow cropping systems.

Estimation of nitrogen supply for winter wheat production through a long-term field trial in China

Excessive synthetic nitrogen (N) applications, high mineral N accumulation and low N use efficiency (NUE) are current issues in intensively cultivated winter wheat production system impeding the sustainable development of agriculture in China. To solve these problems, soil accumulated N in the top 1 m of the soil profile before sowing (N_soil), returned straw-N from the previous maize crop (N_straw) and fertilizer N application (N_fertilizer) should be comprehensively considered N supply sources in N management. As such, the objective of this research was to determine the optimal total N supply (TN_supply) level needed to meet crop requirements while minimizing environmental impacts. A 9-year on-farm experiment was conducted in accordance with a split-plot design involving two different fertilizer management systems (main treatments) and three N application strategies (sub treatments). Extensive TN_supply levels (ranging from 61 kg ha^-1 to 813 kg ha^-1) were detected, and relative yield (RY), N input and N output in response to the TN_supply were measured. The relationships between TN_supply and RY, N input, and N output strongly fit linear-plateau, linear, and linear-plateau models, respectively. The minimum TN_supply levels needed to achieve the maximum RY and N output were 325 and 392 kg ha^-1, respectively. On the basis of N supply capacity, the TN_supply was removed from the growing system by 61% (N input). As the N input increased past 209 kg ha^-1, the NUE declined, at which point the TN_supply reached 433 kg ha^-1. Therefore, the suitable TN_supply should range from 325 kg ha^-1 (ensuring a total N supply for high yield and N uptake) to 433 kg ha^-1 (obtaining a relatively higher NUE and less N loss to the environment). The TN_supply was highlighted to be an indicator for use in N management recommendations. Considering the average high N accumulation in winter wheat production systems, N management should essentially take into account the consumption of N_soil, the levels of N_straw and the minimum application of N_fertilizer to obtain high yields while minimizing environmental impacts under suitable TN_supply levels.

Influence of irrigation with microalgae-treated biogas slurry on agronomic trait, nutritional quality, oxidation resistance, and nitrate and heavy metal residues in Chinese cabbage

Biogas slurry (BS) is a main byproduct of biogas production that is commonly used for agricultural irrigation because of its abundant nutrients and microelements. However, direct application of BS may cause quality decline and nitrate and heavy metal accumulation in crops. To address this issue, a microalgae culture experiment and an irrigation experiment were performed to evaluate the removal efficiencies of nutrients and heavy metals from diluted BS by microalgae Scenedesmus sp. and to investigate the effects of irrigation with microalgae-treated BS (MBS-25, MBS-50, MBS-75, and MBS-100) on nutritional quality, oxidation resistance, and nitrate and heavy metal residues in Chinese cabbage. After 8 days of continuous culture, a ratio of 1/1 for BS/tap water mixture (BS-50) was the optimal proportion for microalgal growth (3.73 g dry cell L^-1) and efficient removal of total nitrogen (86.1%), total phosphorus (94.3%), COD (87.5%), Cr (50%), Pb (60.7%), and Cd (59.7%). The pH in MBS-50 medium recovered to the highest level in a shorter period of time and accelerated the gas stripping of ammonia nitrogen and the formation of insoluble phosphate and metals, which partly contributed to the high removal efficiencies. MBS irrigation significantly promoted crop growth; improved nutritional quality, edible taste, and oxidation resistance; and reduced nitrate and heavy metal residues in Chinese cabbage at a large scale. Therefore, microalgae culture was beneficial to reduce negative impacts of BS irrigation in crop growth and agricultural product safety. This study may provide a theoretical basis for the safe utilization of BS waste in agricultural irrigation.

Biogas residues in substitution for chemical fertilizers: A comparative study on a grassland in the Walloon Region

To provide sufficient quantities of food and feed, farming systems have to overcome limiting factors such as the nutrient depletion of arable soils. Nitrogen being the main mineral element required for plant growth, has led to the extensive use of chemical fertilizers causing nitrogen pollution of the ecosystems. This field study investigates the use of biogas residues (BRs) as biofertilizers and their contribution to the mitigation of nitrate leaching in agricultural soils, while also demonstrating the polluting nature of chemical fertilizers. Nine different fertilization treatments classified in three schemes and two nitrogen doses were tested for three consecutive years on a grassland in the Walloon Region of Belgium. Residual soil mineral nitrogen, percentage contribution of treatments in residual nitrate and agronomic performance were assessed for each fertilization treatment. The results obtained showed significant differences on treatment and scheme level regarding nitrate accumulation in the soil, with chemical fertilizers posing the highest nitrate leaching risk. BRs did not cause nitrate accumulation in the soil, and were N rate and rainfall independent, while the chemical treatments indicated a cumulative tendency under high N rate and low precipitation. Forage yield did not demonstrate statistical differences on treatment and scheme level but varied with changing precipitation, while the maximum application rate suggested a plateau. Aboveground nitrogen content was significantly higher after the application of chemical fertilizers only in the first year, while all the chemical treatments indicated a dilution effect under elevated annual rainfall. Finally, the partial substitution of chemical fertilizers by raw digestate reduced the concentration of NO₃^- in the soil without having a negative impact on the yield and N content of the biomass. These results strongly advocate for the environmental benefits of BRs over chemical fertilizers and underline their suitability as biofertilizers and substitutes for chemical fertilizers in similar agricultural systems.