NIAB's Forage Crop Specialist Ellie Roberts explains about the first phase of a project involving NIAB and the Maize Growers’ Association (MGA), which is investigating soil nitrogen (N) release in relation to levels of soil organic matter at three maize sites.
The Nitrogen through the growing season project started in 2023 focusing on improving understanding of Soil Nitrogen Supply (SNS) and Nitrogen Use Efficiency (NUE) in UK maize crops in order to assess the quantity and timing of nitrogen released from different soils and different growing conditions.
This MGA/NIAB initiative was supported by Wessex Water, Catchment Sensitive Farming Natural England), Yara UK, KWS, Limagrain and Grainseed Ltd and is planned to continue at a larger scale in 2024.
A combination of desktop research, nitrogen response trials, overwinter nitrogen leaching recording trials, soil nitrogen testing (including Additional Available Nitrogen (AAN)) and consultation with farmers and industry identified a greater potential for soil to supply the N requirements of growing maize crops, particularly in higher organic matter soils.
Soil nitrogen release is influenced by soil type, moisture, temperature, organic matter and soil biology, with N availability increasing through June to August and maize N demand increasing during this same period. Increased understanding of the rate of soil N release during the maize growing season should allow better matching of applied N to crop need, improving Nitrogen Use Efficiency (NUE) reducing costs and associated environmental impacts.
During the 2023 growing season there were three work streams:
- A literature review to improve understanding of soil nitrogen release during the maize growing season
- Soil nitrogen testing (including Additional Available Nitrogen) undertaken at three sites at regular intervals through the growing season
- Foliar (leaf) maize tissue analysis was undertaken to ascertain nitrogen content periodically throughout the growing season.
Nitrogen response curves were plotted for the Somerset and Norfolk sites as part of the wider MGA research programme. Data from the Somerset trial found no yield, maturity, or quality benefit from applying inorganic nitrogen, the SNS supply providing all the crop needed.
This was found to be common in moist, warm, high organic matter soils. SNS in the Somerset trial was high (>300 kg/ha). The Norfolk site showed a yield and quality benefit of applying inorganic nitrogen post drilling, suggesting that the SNS alone was not sufficient to meet crop needs on the lower organic matter soil. SNS at the Norfolk site was the lowest of the three sites peaking at 130 kg N/ha.
In the UK, the period of rapid maize growth occurs when soils are warm and usually still moist. Soil N mineralisation is also influenced by temperature and moisture.
It is important to note that as well as rate of mineralisation, N uptake also depends on soil moisture and temperature, which heavily influences the amount and rate of microbial activity as well as plant physiology, soil organic matter, crop residues and tillage. Data on mycorrhizal root systems indicates these fungi have an important role in retaining nutrients in the soil organic matter (SOM), reducing leaching.
Efficient maize genotypes have been found to develop a greater root-to-shoot ratio and undergo a slower rate of phenological development, with a greater proportion of root biomass enhancing the N absorption capacity.
Tillage system has also been found to influence nutrient uptake. Tillage system, soil and weather factors should also be factored into N fertiliser recommendations, crop rotation and tillage alter SOM dynamics and C and N cycling with changes in soil organic carbon (SOC) and total N. Mineralisation of SOM can supply a significant proportion of N required by maize with amount of N supplied by SOM ranging from 60 to 130 kg N/ha depending on soil type, conditions and N stocks.
Organic matter also impacts rooting and soil warming along with many other influential factors. Organic matter in maize soils can act as a temporary N sink, reducing leaching from fertilisation and releasing nitrates in following growing seasons.
Mineralisation
Gross N mineralisation measures the rate of inorganic ammonium production by soil organisms whereas net N mineralisation measures change in the size of soil inorganic N pools over time in the absence of roots. The supply rate of N from mineralisation of organic matter is limited by the rate of microbial mineralisation, which may then be subject to nitrification, immobilisation into the microbial biomass, and uptake by plants.
Undersowing and companion sowing maize with legumes, is increasing in order to improve NUE and provide soil cover after maize harvest. Incorporation of legume residues and animal manures have also been shown to increase net N mineralisation.
Both gross ammonification and net N mineralisation rates are enhanced in high organic matter soils and management practices that increase labile organic matter, such as manuring, have been shown to enhance gross ammonification rates.
Additionally, maize plant root activity has been found to enhance N mineralisation in the rhizosphere and thus the supply of N from organic N pools. Inorganic N produced by gross ammonification of soil organic N has been found to be relatively high compared to maize uptake.
Meeting maize N demand during periods of peak uptake with soil organic N therefore depends on the ability of the crop to compete with other ammonia consumption pathways which depends on the microbial immobilisation demand, nitrification and denitrification rates, as well as soil characteristics that determine the mobility of N (porosity, moisture, clay content), root morphology, physiology and N uptake efficiency.
Maize N uptake has been found to account for only a small proportion of the potential rate of soil N mineralisation. This suggests microbial production of ammonia could be sufficient to meet maize N demand in some soils during warmer months when taking into account small inorganic N pools with high turnover rates, as well as potential for maize to compete with microbes for mineralised N. Trial data from high SOM sites indicates that SNS from these soils can meet the N requirement of high yielding maize crops.
Conventional agricultural N management strategies assume roots compete poorly with soil microbes for inorganic N, only accessing N which is in excess of microbial demand so current N strategies largely rely on inorganic N fertiliser additions to achieve high levels of crop production and fail to account for SNS in maize ground to achieve synchrony between N demand and supply.
With high organic matter soils likely to meet maize N requirements through mineralisation of soil N during the period of rapid growth, the need for additional artificial N at that time is negated, contrary to current maize fertiliser N recommendations. This suggests that a review of the recommendations, taking soil organic matter into account is necessary with potential for both financial and environmental benefits.
By continuing to investigate the inorganic N supply capacity of the soil in 2024, we hope to help address the current asynchrony between maize N supply and demand in higher OM soils.
This article originally appeared in the Spring 2024 edition of NIAB’s Landmark magazine. Landmark features in-depth technical articles on all aspects of NIAB crop research, comment and advice. You can sign up for free and get Landmark delivered to your door or inbox: