Nutrient Cycling in Agroecosystems

Abstract In this work, the hypothesis that stabilized nitrogen fertilizers (SNF) might be useful to boost paddy rice production with a reduced impact on nitrous oxide (N2O) and methane (CH4) emissions from soil was tested by conducting a field experiment on a subtropical typic Albaqualf in Southern Brazil over two rice growing seasons (GS). Treatments comprised the use of no nitrogen fertilizer (control) and application of common urea, urea + NBPT [urea plus the urease inhibitor N-(n-butyl) thiophosphoric triamide], urea + NBPT + DCD (urea plus NBPT and the nitrification inhibitor dicyandiamide), urea + Cu B (urea plus copper and boron), urea + S (urea plus sulfur) and urea + zeolite (urea plus zeolite). Field plots were arranged according to completely randomized block design. Methane and N2O fluxes were measured by using the static chamber technique and gases analyzed by gas chromatography. Seasonal partial global warming potential (pGWP) values were calculated as the combination of the seasonal levels for the individual gases multiplied by the respective radiative forcing potentials (viz., 28 for CH4 and 265 for N2O), and yield-scaled greenhouse gas (GHG) emissions were taken to be the ratios of pGWP to rice grain yields. N2O emissions were 0.9 kg ha−1 on average in GS1; also, they were similar irrespective of treatment and whether or not N fertilization was applied. In GS2, N2O emissions were lowest with the control treatment and all SNF led to emission levels comparable to those of common urea (average 4.5 kg ha−1). CH4 emissions in GS1 peaked at 474 kg ha−1 and exhibited no significant differences among fertilizers. On the other hand, urea + NBPT reduced CH4 emissions relative to urea + S and urea + zeolite in GS2. pGWP averaged at 11 216 kg eq.CO2 ha−1 across the two GS without N fertilization (control treatment) and at 11 803 kg eq.CO2 ha−1 with fertilization. The increase in grain yield resulting from nitrogen fertilization was similar irrespective of nitrogen source, and averaged at 9300 kg ha−1 in GS1 and 8200 kg ha−1 in GS2. Yield-scaled pGWP was only influenced by fertilization in GS1, where urea + NBPT and urea + zeolite reduced it relative to the control treatment. The starting hypothesis was thus rejected since SNF affected rice grain yield and soil GHG emissions similarly to common urea over two growing seasons in subtropical paddy rice ecosystems of southern Brazil.

Abstract The Green Revolution rapidly increased India’s food production since the 1960s, but excessive synthetic fertilizer use caused severe environmental problems. Our spatially explicit analysis for 1970–2020 indicates an uneven distribution of the dramatic increase of surpluses of India’s soil N (4.3 to 21.6 Tg N/year) and P budget (0.4 to 3.3 Tg P/year): with high surpluses in e.g., Green Revolution (GR) and South-West (SW) regions, lower surpluses in e.g., North-West (NW) and even local deficits within some other states Nutrient surpluses were growing primarily through increased use of synthetic fertilizers, which resulted in declining nutrient use efficiency, high N and P losses and soil P accumulation, with large regional heterogeneity. Total N loss is determined by the surplus, through “holes in the pipe”, whereby the holes (loss pathways) differ in diameter, as determined by climate, crop, soil, terrain and management. Ammonia volatilization ranged from 16% of the N surplus (NC) to 45% (GR states), and denitrification losses from 50% (NC and SW) to 38% (GR) and 28% (NE). N loss via leaching ranged from 46% (NE), to 22–26% (GR and NC), and 16% (SW), and surface runoff losses between 2% (GR) and 9% (NC). In the period 1970-2020, our estimated soil P accumulation across India amounts to 290 kg P/ha, highest in SW followed by NE and GR states and lowest in NC. The SW region also has the highest surface runoff P loss (40% of its regional budget), followed by NC (53%), NE (34%), and GR (26%). Addressing these regional differences can help developing effective, targeted and region-specific nutrient management strategies while meeting India’s rising food demand.

Abstract Mechanical weed control is a major element of weed suppression in organic farming systems. In addition to the direct effect on weed growth, mechanical weeding, such as harrowing or hoeing, is known to induce side effects on several soil- and crop-related properties. In this study, we investigated the impact of mechanical weeding on soil mineral nitrogen (SMN), soil moisture, and crop yield in an organic crop rotation of grass-clover (Lolium multiflorum Lam., Trifolium pratense L.), silage maize (Zea mays L.) and winter barley (Hordeum vulgare L.). The experiment was conducted in two consecutive years (2021, 2022), where each crop was grown in each year on a Plaggic Anthrosol with sandy loam in North-West Germany. Two weed control treatments (mechanical: harrowing, hoeing; chemical: herbicide application) were implemented in a randomized block design with four replications. Greater net nitrogen (N) mineralization in maize compared to winter barley were attributed to the incorporation of grass-clover residues before sowing of maize and greater mineralization potential during the maize growing season. Higher weed growth in maize after mechanical weeding resulted in a reduction of up to 47% in SMN content in the topsoil. In barley, no differences in weed suppression were observed between the treatments and only small effects on SMN were determined after mechanical weeding. The soil water content in the mechanically weeded plots was significantly higher at several events in both years and for both crops, which was attributed to increased water infiltration by disrupting the soil crust. Neither crop yield nor N uptake in harvest products was affected by the different treatments.

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.