Nutrient Cycling in Agroecosystems

Abstract Maize (Zea mays L.), a multi-utility cereal crop, has a high requirement for nitrogen (N). However, conventional N application methods fail to achieve potential yield and contribute to environmental pollution. Thus, a 2-year (2022 and 2023) field investigation was conducted to understand the effect of sub-surface drip-fertigation (SSDF) and legume residue incorporation (3 t ha−1) on improving maize yield, uptake, remobilization, and use-efficiency of N. Four N levels, viz., 0, 50, 75, and 100% recommended dose of N (RDN) applied in 3-splits (3S) and 4-splits (4S), were compared with conventional maize cultivation, fertilized with 150 kg N ha−1 (CMC-N150). Variable N fertigation with SSDF significantly affected the dry matter (DM) and N remobilization from leaves towards kernel; at lower doses of N maize tended to mobilize higher DM and N towards kernel. Better growth, higher nutrient uptake, and better remobilization efficiency of DM/N led to higher grain yield under 75% RDN (19–23% increase) and 100% RDN (20–25% increase) sub-surface drip-fertigated, compared to CMC-N150. Using 50% RDN through SSDF yielded grain on par with CMC-N150, thus saving 50% N. Moreover, applying N in 4-splits (4S) gave 3–5% yield advantage over 3-splits. Partial factor productivity of N (PFPN) and agronomic N-use efficiency (AEN) of different SSDF treatments increased by 18–100% and 40–99%, respectively over CMC-N150. Greengram residue incorporation improved maize yield by 5–8%, PFPN by 4–5% over no-residue incorporation. Overall, there was a 25–50% N economy with concomitant yield improvement under SSDF over conventional N management. The SSDF coupled with legume residue incorporation could be a promising technology for enhancing N-use efficiency globally; however, it needs policy support for adoption in the producer’s field.

Abstract Cover cropping is a common practice among organic growers, well-known for its potential to supply nitrogen (N) to subsequent cash crops. Uncertainties and challenges exist in understanding how cover crops interact with soil properties and management practices across organic farms to supply N, and if such N supply is synchronous with subsequent cash crop N demand. An on-farm study examined cereal rye (Secale cereale) versus crimson clover (Trifolium incarnatum) planted before corn (Zea mays L.) in five organic farms in Michigan with a range of soil properties and management practices. High quality crimson clover residue [Carbon (C):N ratio 15:1] was associated with higher soil inorganic nitrogen, corn chlorophyll content, tissue N content, and grain yields relative to low quality cereal rye residue (C:N ratio 25:1). There were several lines of evidence that low quality cereal rye residue coupled with substantial biomass and a dry season limited N release during peak corn N demand. Nitrogen uptake efficiency (NUE, ratio of total N removed by corn to total N input) was above 1 for corn farms with low soil organic matter (SOM), active N and C pools, and lower than 1 for farms with high SOM and active N and C pools. Overall, cover crop biomass and cover crop quality was a more important driver of corn performance than background SOM content in organic corn farms. Our research highlights the challenges of ensuring sufficient N supply in organic field production, and the importance of planting a legume cover crop before corn.

Abstract With emerging water scarcity and rising fertilizer prices, optimising future water use while maintaining yield and nutrient efficiency in irrigated rice is crucial. Alternate wetting and moderate soil drying irrigation (i.e., re-irrigation when the water level reaches 15 cm below the soil surface) has proven to be an efficient water-saving technology in semi-arid zones of West Africa, reducing water inputs without yield penalty. Alternate wetting and severe soil drying (AWD30), by re-irrigating fields only when the water table reaches 30 cm below the soil surface, may further reduce water inputs compared to farmers’ irrigation practices (FP). However, acute soil drying may impair fertilizer use efficiency and reduce the bio-availability of some key nutrients. This study assessed the potentials and risks associated with AWD30 for smallholder rice farmers in the semi-arid zones of West Africa. We conducted 30 on-farm field trials over three seasons (wet and dry seasons of 2019, and dry season of 2020), in Kou Valley, Burkina Faso. We assessed yield, water productivity, nutrient uptake, and use efficiency under AWD30 and FP. In FP, farmers maintained their fields submerged as frequently as possible according to the scheme-dependent water provision schedule. With AWD30, irrigation frequency was reduced by 30%, however, soils were seldom completely dried due to a shallow groundwater table. Compared to FP, AWD30 reduced irrigation water input by 37% with no significant effects on grain yields (average of 4.5 Mg ha−1), thus increasing average water productivity by 39%. Both irrigation management practices provided comparable crop uptake of N, P, and K, and use efficiencies of applied N and P. However, the N content in straw and the P concentration in grain generally increased with total water input (rain + irrigation). We conclude that at locations with a shallow groundwater table, AWD30 can effectively save irrigation water without significantly reducing the grain yield and the use efficiency of applied mineral nutrients.

Abstract Phosphorus (P) is an essential constituent of life but large P losses from agroecosystems and sanitation systems are a major source of eutrophication in water bodies. These losses are doubly negative as P in human excretions can be used for crop fertilization. Using a unique dataset of 20,000 French WasteWater Treatment Plants (WWTPs) operational measurements over two decades and a P mass balance of the sanitation system, we assess the fate of human excretions and their agricultural potential. Despite 75% of French WWTPs sludge being applied to crops, only 50% of the excreted P is returned to agroecosystems. This is among the highest rate in Western countries. Meanwhile, another 35% of the excreted P ends up in surface waters or the terrestrial environment through WWTP discharge, diffuses losses from individual autonomous systems, and sewers leaks. The remaining 15% is contained in sludge that is incinerated or sent to landfills. Moreover, while WWTP removal efficiency increased in the 2000s, reaching 80% on average nationally, it has been followed by a decade of stagnation in every French basin. The final removal efficiency for each basin (65% to 85%) closely correlates with how much of the basin area is defined as P-sensitive in the European directive. Our results suggest that recycling all P in excretions could help supply 7 to 34% of the P demand in French food supply, without changing the current food system. Reshaping agricultural systems (shifting to more plant-based diets, decreasing P losses and food waste) would enable to go even further on the road to food sufficiency.