Estimating annual soil carbon loss in agricultural peatland soils using a nitrogen budget approach.

Around the world, peatland degradation and soil subsidence is occurring where these soils have been converted to agriculture. Since initial drainage in the mid-1800s, continuous farming of such soils in the California Sacramento-San Joaquin Delta (the Delta) has led to subsidence of up to 8 meters in places, primarily due to soil organic matter (SOM) oxidation and physical compaction. Rice (Oryza sativa) production has been proposed as an alternative cropping system to limit SOM oxidation. Preliminary research on these soils revealed high N uptake by rice in N fertilizer omission plots, which we hypothesized was the result of SOM oxidation releasing N. Testing this hypothesis, we developed a novel N budgeting approach to assess annual soil C and N loss based on plant N uptake and fallow season N mineralization. Through field experiments examining N dynamics during growing season and winter fallow periods, a complete annual N budget was developed. Soil C loss was calculated from SOM-N mineralization using the soil C:N ratio. Surface water and crop residue were negligible in the total N uptake budget (3 - 4 % combined). Shallow groundwater contributed 24 - 33 %, likely representing subsurface SOM-N mineralization. Assuming 6 and 25 kg N ha-1 from atmospheric deposition and biological N2 fixation, respectively, our results suggest 77 - 81 % of plant N uptake (129 - 149 kg N ha-1) was supplied by SOM mineralization. Considering a range of N uptake efficiency from 50 - 70 %, estimated net C loss ranged from 1149 - 2473 kg C ha-1. These findings suggest that rice systems, as currently managed, reduce the rate of C loss from organic delta soils relative to other agricultural practices

Introduction of a Fallow Year to Continuous Rice Systems Enhances Crop Soil Nitrogen Uptake

Rice grown in California constitutes 20% of total U.S. rice production and is typically grown in a continuous rice monoculture system. In recent years, growers have been forced to fallow their lands often due to winter droughts leading to water restrictions or spring rains leading to prevented planting. Increased soil aeration due to fallowing creates knowledge gaps in soil nitrogen (N) availability. A two-year field study was conducted to evaluate differences in crop N uptake between rice cultivation following a fallow season, fallow rice (FR) and continuous rice (CR) systems. Crop uptake of soil N (N uptakesoil) and fertiliser N (N uptakefertilizer) were quantified using 15N-enriched ammonium sulfate applied in microplots as a preplant (150 kg N ha−1) or topdress (30 kg N ha−1) application. In both seasons when N was applied as a preplant fertiliser, the FR treatment had a higher grain yield than did the CR treatment, with yield differences of 2.3 Mg ha−1 in 2021 (p < 0.05) and 1.7 Mg ha−1 in 2022 (p < 0.05). Examining the sources of crop N uptake for preplant applied N, on average, N uptakesoil in the FR treatment was 16.7 kg N ha−1 higher than the CR treatment at maturity (p < 0.05). In contrast, N uptakefertilizer was similar between treatments. Additionally, comparable soil and crop fertiliser N recoveries in CR and FR preplant N suggested that the pathways and magnitudes of fertiliser N losses were similar in both systems. These results indicate that N uptakesoil was primarily responsible for lower N uptake in CR. Similar results were found when N was applied as a topdress, where FR had increased N uptakesoil in both years. We further investigated the reason for lower rates of N uptakesoil in CR. Soil phenols, which have been documented to accumulate in continuously flooded rice systems and stabilise soil N, were quantified in the field study. Complementing the rigorous field study, a regional survey study that incorporated nine paired fields was conducted to quantify regional phenol levels. In both the field and the regional survey studies, soil phenols were higher in CR than in FR fields. Together, higher phenol levels and lower N uptakesoil in CR provide mechanistic evidence that the introduction of a season-long fallow to continuous rice systems enhances soil N availability by reducing organic substrate recalcitrance. Future work should identify the duration needed for soil phenol accumulation to impair soil N cycling under continuous rice cultivation, as well as any roles of soil microbial populations in these soil N cycling patterns

The Organic Rice Production System in California

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Estimating yield potential in temperate high-yielding, direct-seeded US rice production systems

Accurate estimation of a crop’s yield potential (Yp) is critical to addressing long-term food security via identification of the exploitable yield gap. Due to lack of field data, efforts to quantify crop yield potential typically rely on crop models. Using the ORYZA rice crop model, we sought to estimate Yp of irrigated rice for two widely used rice varieties (M-206 and CXL745) in three major US rice-producing regions that together represent some of the highest yielding rice regions of the world. Three major issues with the crop model had to be addressed to achieve acceptable simulation of Yp; first, the model simulated leaf area index (LAI) and biomass agreed poorly for all direct-seeded systems using default settings;second, cold-induced sterility and associated yield losses were poorly simulated for environments with a large diurnal temperature variation; lastly, simulated Yp was sensitive to the specified definition of physiological maturity. Except for the simulation of cold-induced sterility, all issues could be remedied within the existing model structure. In contrast, simulation of cold-induced sterility posed a continuing challenge to accurate simulation—one that will likely require changes to ORYZA’s formulation. Estimates of Yp from the modified model were validated against large multi-year data sets of experimental yields covering the majority of US rice production areas. Validation showed the adjusted model simulated Yp well, with most top yields falling within 85% of Yp for both varieties (77% and 78% observed yields within15% of Yp for CXL745 and M-206 respectively). Maximum estimated Yp was 14.3 (range of 8.2–14.5) and14.5 (range of 8.7–15.3) t ha−1for the Southern US and CA, respectively

Crop Rotations in California Rice Systems: Assessment of Barriers and Opportunities

Flooded rice soils are unique in terms of maintaining soil fertility and long-term productivity, allowing continuous rice systems to contribute greatly to global food supply. Yet increasing herbicide resistant weed pressure, water scarcity, and other sustainability challenges suggest a need to explore options for cropping system diversification. However, little research has evaluated the current obstacles limiting diversification of rice systems in different contexts. During summer and fall of 2020 we interviewed 42 rice growers to (i) assess the perceived benefits and challenges of crop rotation in the context of California rice systems and (ii) identify the factors influencing decision-making and barriers to adoption. Rice-based cropping systems ranged from high to low diversity across three different categories of growers (conventional rotations > organic > continuous rice). Key factors influencing the feasibility of rotations were soil limitations, production costs and productivity level of alternative crops, water and equipment requirements, market access, and regional differences. Generally, growers agreed that weed control and reduced reliance on herbicides were benefits of rotation. Similarly, growers who rotated described soil health as a primary benefit that decreases the need for fertilizer and pesticide inputs. However, there were many challenges to implementing rotations including heavy clay soils with poor drainage, lack of available contracts and markets for other crops, financial barriers such as land ownership and farm infrastructure (size of operation and available labor and equipment), and limited experience and knowledge of other viable crops. In terms of economic feasibility, those who only grow rice believed that other crops are less profitable, while those who rotate said that rotations increased profitability. Our research indicates that soil conditions are an important limitation, but other economic, social, and cultural barriers also strongly influence the potential for the diversification of rice systems

Switchgrass is a promising, high-yielding crop for California biofuel

Ethanol use in California is expected to rise to 1.62 billion gallons per year in 2012, more than 90% of which will be trucked or shipped into the state. Switchgrass, a nonnative grass common in other states, has been identified as a possible high-yielding biomass crop for the production of cellulosic ethanol. The productivity of the two main ecotypes of switchgrass, lowland and upland, was evaluated under irrigated conditions across four diverse California ecozones - from Tulelake in the cool north to warm Imperial Valley in the south. In the first full year of production, the lowland varieties yielded up to 17 tons per acre of biomass, roughly double the biomass yields of California rice or maize. The yield response to nitrogen fertilization was statistically insignificant in the first year of production, except for in the Central Valley plots that were harvested twice a year. The biomass yields in our study indicate that switchgrass is a promising biofuel crop for California

Sustainable intensification for a larger global rice bowl

Future rice systems must produce more grain while minimizing the negative environmental impacts. A key question is how to orient agricultural research & development (R&D) programs at national to global scales to maximize the return on investment. Here we assess yield gap and resource-use efficiency (including water, pesticides, nitrogen, labor, energy, and associated global warming potential) across 32 rice cropping systems covering half of global rice harvested area. We show that achieving high yields and high resource-use efficiencies are not conflicting goals. Most cropping systems have room for increasing yield, resource-use efficiency, or both. In aggregate, current total rice production could be increased by 32%, and excess nitrogen almost eliminated, by focusing on a relatively small number of cropping systems with either large yield gaps or poor resource-use efficiencies. This study provides essential strategic insight on yield gap and resource-use efficiency for prioritizing national and global agricultural R&D investments to ensure adequate rice supply while minimizing negative environmental impact in coming decades

Spatiotemporal variability of soil fertility and nutrient uptake in rice soils: the role of flood water movement

Póster presentado en la 33th Rice Technical Working Group meeting (RTWG 2010), celebrada en Biloxi (Estados Unidos) del 22 al 25 de febrero de 2010.Understanding the mechanisms driving spatiotemporal variability of yield within a field is necessary in order to predict and effectively manage it. We are investigating the relative importance of three management-driven factors - flood water movement, water temperature and laser-leveling - in driving spatiotemporal variability of yield across field scales.Financial support was provided by the Kearney Foundation.Peer Reviewe

Water balances and evapotranspiration in water and dry‐seeded rice systems

Rice is a crop that is usually grown under flooded conditions and can require large amounts of water. The objective of this 3-year study was to quantify water use in water- (WS) and dry-seeded (DS) systems. In WS systems, the field is continuously flooded, while in DS systems the field is flush irrigated for the first month and then flooded. Research was conducted on commercial rice fields where the residual of the energy balance method using a sonic anemometer and the eddy covariance method were used to determine crop evapotranspiration (ETc) and crop coefficient (Kc) values. In addition, inlet irrigation water and tailwater drainage were determined. Across years, there was no difference in ETc (averaged 862 mm), sea- sonal Kc (averaged 1.07), irrigation water delivery (aver- aged 1839 mm) and calculated percolation and seepage losses (averaged 269 mm) between systems. An analysis of the first month of the season, when the water management between these two practices was different, indicated that Kc and water use were lower in DS systems relative to WS systems when there was only one irrigation flush during this period, while two or three irrigation flushes resulted in similar values between the two systems