Addressing nitrogen and water availability challenges in semi-arid maize cropping systems

Abstract

In many parts of the world, access to irrigation is threatened as competition for water resources increases and water availability decreases. This includes the Great Plains Region of the U.S. where roughly 25% of U.S. irrigated cropland is located. Loss of irrigation threatens global food security as irrigated lands are highly productive; accounting for just 20% of cropland but responsible for 40% of agricultural production. Thus, there is urgent need for achieving high yields with less water. Many studies have been conducted to increase crop productivity with limited water, but the interactive effect of nitrogen (N) and water availability on crop response has received limited attention with variable conclusions. Additionally, the effect of varying N availability under different water levels on soil N mineralization (Nmin), contribution of Nmin to crop N uptake, and the recovery and fate of N fertilizer has been largely unexplored. Soil Nmin is an important source of N for crops therefore, quantifying Nmin rates and contribution to crop N uptake is important for N management. Additionally, minimizing N losses is an important goal for agroecosystems as N losses come with an economic and environmental cost. The general aim of my dissertation was to explore the effects of N availability on soil N cycling and crop response within maize cropping systems, an important irrigated crop in the Great Plains Region, under contrasting water availability. Examining field data from 2021 – 2023, I found that maize grain yield response to N was dependent on water. When water was limited, maize grain yield was maximized with ~ 200 kg N ha-1, with excess N being detrimental for all three growing seasons. This was true even during 2023, which was an extremely wet year, and had reduced N fertilizer rates due to higher pre-plant soil residual N. Maize N uptake continued to increase with N availability beyond 200 kg N ha-1, showing maize was not co-limited by N when water was limited water. Rather, excess N and subsequent N uptake had negative effects on root and shoot growth, potentially via effects on stomatal conductance and photosynthesis, leading to yield declines. Soil net Nmin surrounding peak maize N uptake exhibited an N × water interaction where increased N fertilization rate decreased net Nmin with full water but increased with N rate when water was limited. Soil N-acquiring enzyme activity, a proxy for gross Nmin, had a different response where it increased with N regardless of water. This could suggest N fertilizer increased plant available N through increased microbial mediated depolymerization of N containing compounds in the soil. The different responses were likely due to the exclusion of living maize plants and maize N uptake in the net Nmin incubation tubes. Across the entire season, both net Nmin and enzyme activity tended to be higher during maize vegetative stages than during early reproductive stages when N demands are the highest. A 15N tracer study revealed that recovery was high, and losses low and unaffected by N and water treatments. This suggests that lower N rates should have lower N losses. Maize N uptake increased with N rate, but primarily from 15N fertilizer, rather than non-N fertilizer sources such as soil Nmin. This could be due to the asynchrony between soil N supply and maize N demand. Additionally, microbial biomass N at the end of the season suggests that immobilization occurred, but primarily for non-N fertilizer sources. Immobilization of non-N fertilizer sources later in the season when soil Nmin rates are low and maize N demands are high likely led to maize acquiring N from N fertilizer to meet its N requirements. The wet growing season during 2023 made the water treatments negligible. Future studies with treatment differences in water availability could reveal how water availability affects fate and recovery of N fertilizer, as well as contribution of non-N fertilizer sources, such as soil Nmin, to crop N uptake with different N fertilizer rates. Overall, my findings show that water limited maize is not co-limited by N, and that excess N is detrimental to maize growth and yield. For limited water, reducing N fertilizer rate should reduce N losses while still maximizing yields and resource use efficiency. Reducing N fertilizer rate when water is fully available and maize N demands are high may be challenging. Higher N fertilizer rates appeared to increase bioavailable N through increased soil enzyme activity, however maize was not able to significantly increase uptake of N from sources other than the 15N fertilizer applied, such as soil Nmin, regardless of treatment. Maize was more reliant on N fertilizer rather than non-N fertilizer sources when supplied with high N fertilizer rates, while more reliant on non-N fertilizer sources when supplied with low N fertilizer rates. Management practices that increase internal N cycling, especially later in the season when maize N demands are greater, may help reduce the reliance on synthetic N fertilizer inputs thus reducing N losses without sacrificing productivity. Using field experiments from 2021 – 2023 I found that maize grain yield response to N was dependent on water. When water was limited, maize grain yield was maximized with ~ 200 kg N ha-1, with excess N being detrimental for all three growing seasons. This was true even during 2023, which was an extremely wet year, and had reduced N fertilizer rates due to higher pre-plant soil residual N. With limited water, maize N uptake continued to increase with N availability beyond 200 kg N ha-1, showing maize was not co-limited by N. Rather excess N and subsequent N uptake had negative effects on root growth, and potentially stomatal conductance and photosynthesis leading to yield declines. Soil net Nmin surrounding peak maize N uptake exhibited an N × water interaction where increased N fertilization rate decreased net Nmin with full water but increased with N rate when water was limited. Soil N-acquiring enzyme activity, a proxy for gross Nmin, had a different response where it increased with N regardless of water. This could suggest N fertilizer increased plant available N through increased microbial mediated depolymerization of N containing compounds in the soil. The different responses were likely due to the exclusion of living maize plants and maize N uptake in the net Nmin incubation tubes. For the entire season both net Nmin and enzyme activity tended to be higher during maize vegetative stages as opposed to early reproductive stages when N demands are the highest. A 15N tracer study revealed that recovery was high, and losses were low and unaffected by the N and water treatments. This suggests that lower N rates should have lower N losses. Maize N uptake increased with N rate, but primarily from 15N fertilizer, rather than non-N fertilizer sources such as soil Nmin. This could be due to the asynchrony between soil N supply and maize N demand. Additionally, microbial biomass N at the end of the season suggests that immobilization occurred and primarily occurred for non-N fertilizer sources. Immobilization of non-N fertilizer sources later in the season when soil Nmin rates are lower, and maize N demands are high, likely led to maize acquiring N from N fertilizer to meet its N requirements. The wet growing season during 2023 made the water treatments negligible. Future studies with more extreme water differences will help reveal how water availability affects fate and recovery of N fertilizer, as well as contribution of non-N fertilizer sources, such as soil Nmin, to crop N uptake with different N fertilizer rates. Overall, my findings show that water limited maize is not co-limited by N, and that excess N is detrimental to maize growth and yield. For limited water, reducing N fertilizer rate should reduce N losses while still maximizing yields and resource use efficiency. Reducing N fertilizer rate when water is fully available and maize N demands are high may be challenging. Higher N fertilizer rates appeared to increase bioavailable N through increased soil enzyme activity, however maize was not able to significantly increase uptake of N from sources other than the 15N fertilizer applied, such as soil Nmin, regardless of treatment. Maize was more reliant on N fertilizer rather than non-N fertilizer sources at the higher N fertilizer rate, while the lower N fertilizer rates were more reliant on non-N fertilizer sources. Management practices that increase internal N cycling, especially later in the season when maize N demands are greater, may help reduce the reliance on synthetic N fertilizer inputs thus reducing N losses without sacrificing productivity.