Non-growing Season Precipitation Research Articles

Regulatory effects of non-growing season precipitation on the community structure, biomass allocation, and water-carbon utilization in a temperate desert steppe

Under the influence of global change, precipitation amounts and extreme precipitation frequency during non-growing seasons in mid-high latitude grasslands have been increasing. However, the ecological effects of non-growing season precipitation in the desert steppe have long been overlooked due to an insufficient understanding of the correlative mechanisms linking non-growing season precipitation to plant growth. Therefore, a 3-year non-growing season precipitation manipulation experiment was conducted to reveal the response of desert steppe plants to non-growing season precipitation changes. Our study indicates that, by influencing water budget and availability, non-growing season precipitation directly or indirectly impacted community structure, plant biomass allocation, and water-carbon utilization intensity. Adaptive strategies of communities and plants included: ① Dominant species enhanced their dominance in the community to adapt to non-growing season precipitation changes. ② Stipa krylovii exhibited different biomass allocation strategies in response to non-growing season precipitation variations. Plants in the precipitation shading plots tended to allocate biomass to the roots, while those in the precipitation increase plots favored aboveground development. ③ Persistent drought during the growing season intensified early insufficient development of plants in the precipitation shading plots. Upon entering the wet period, plants in the precipitation shading plots shifted into a compensatory growth mode with high water-carbon activity intensity, while those in the precipitation increase plots entered a moderate growth mode with relatively low water-carbon activity intensity. Additionally, our study found that the regulatory effects of non-growing season precipitation were more pronounced in the growing seasons with less precipitation in the early to middle stage. Moreover, increased non-growing season precipitation enhanced plant water use efficiency (WUE) and strengthened their resilience to drought conditions. Our study suggests that the ecological role of non-growing season precipitation may be further highlighted in the future climate change pattern. Given the worldwide increase in frequency of extreme precipitation events, particular vigilance should be paid to the underlying long-term adverse effects of severe droughts during the non-growing season. Our findings provide new insights and valuable experimental observational evidence for the climate change impact assessment and response in xerophytic grassland ecosystems.

非生长季降水对青藏高原高寒草甸优势种生物量稳定性的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 非生长季降水对青藏高原高寒草甸优势种生物量稳定性的影响 DOI: 10.5846/stxb202103200741 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家科学基金（U20A2007-01）；第二次青藏高原综合科学考察研究（2019QZKK0307）；山西省博士毕业生、博士后研究人员来晋工作奖励资金科研项目（SXBYKY2021067）；山西农业大学博士科研启动项目（2021BQ34） Non-growing season precipitation facilitated the biomass stability of dominant species in alpine meadow of the Qinghai Tibet Plateau Author: Affiliation: Fund Project: National Natural Science Foundation of China 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:受全球气候变化的影响，青藏高原在过去的几十年间整体上呈现暖湿化的趋势，相比于年际之间温度和降水的变化外，生长季和非生长季气候变化模式的差异可能会对生态系统产生更重要的影响，但相关的研究尚不充分。以青藏高原东部的高寒草甸为研究对象，基于2001年至2017年17年的野外观测数据，包括优势植物紫花针茅的高度、多度以及生物量、次优势物种洽草的生物量，结合生长季和非生长季平均温度和降水量的变化，通过线性回归以及结构方程模型，探究生长季/非生长季不对称气候变化对于青藏高原高寒草甸优势物种生物量稳定性的影响。研究结果表明：1）青藏高原东部年均温和年降水在过去的17年间显著增加，呈现暖湿化的趋势，但是非生长的降水却变化不明显；2）紫花针茅的高度、多度以及生物量在过去17年没有显著的趋势，但是洽草的生物量稳定性显著减少；3）非生长降水结合紫花针茅的高度、多度以及洽草的生物量稳定性促进了紫花针茅的生物量稳定性。研究结果可以为青藏高原高寒草甸在未来气候变化的背景下合理保护与利用提供科学依据。 Abstract:In the past few decades,the Qinghai-Tibet Plateau showed warming and humidifying with global climate change. Compared with the interannual changes of temperature and precipitation, the difference of climate change patterns between growing season and non-growing season may have a more important impact on the ecosystem, but the relevant research is still insufficient. Based on the field observation data of 17 years from 2001 to 2017, including the height, abundance and biomass of dominant plant Stipa purpurea, and biomass of Subdominant species Koeleria cristata, combined with the changes of mean temperature and precipitation in growing season and non-growing season, the growth season/non growing season was explored by linear regression and structural equation model to explore the effects of asymmetric climate change in growing season and non-growing season on biomass stability of dominant species in alpine meadow of Qinghai Tibet Plateau. The results show that:1) the annual mean temperature and annual precipitation in the eastern Tibetan Plateau have increased significantly in the past 17 years, showing a trend of warming and wetting, but the non-growing precipitation has no significant change; 2) The height, abundance and biomass of Stipa purpurea had no significant change trend in the past 17 years, but the biomass stability of Koeleria cristata decreased significantly; 3) The non-growth precipitation combined with the height and abundance of Stipa purpurea and the biomass stability of Koeleria cristata promoted the biomass stability of Stipa purpurea. The results can provide scientific basis for the reasonable protection and utilization of alpine meadow in the context of climate change in the future. 参考文献 相似文献 引证文献

Plant Species Richness in Multiyear Wet and Dry Periods in the Chihuahuan Desert

In drylands, most studies of extreme precipitation events examine effects of individual years or short-term events, yet multiyear periods (>3 y) are expected to have larger impacts on ecosystem dynamics. Our goal was to take advantage of a sequence of multiple long-term (4-y) periods (dry, wet, average) that occurred naturally within a 26-y time frame to examine responses of plant species richness to extreme rainfall in grasslands and shrublands of the Chihuahuan Desert. Our hypothesis was that richness would be related to rainfall amount, and similar in periods with similar amounts of rainfall. Breakpoint analyses of water-year precipitation showed five sequential periods (1993–2018): AVG1 (mean = 22 cm/y), DRY1 (mean = 18 cm/y), WET (mean = 30 cm/y), DRY2 (mean = 18 cm/y), and AVG2 (mean = 24 cm/y). Detailed analyses revealed changes in daily and seasonal metrics of precipitation over the course of the study: the amount of nongrowing season precipitation decreased since 1993, and summer growing season precipitation increased through time with a corresponding increase in frequency of extreme rainfall events. This increase in summer rainfall could explain the general loss in C3 species after the wet period at most locations through time. Total species richness in the wet period was among the highest in the five periods, with the deepest average storm depth in the summer and the fewest long duration (>45 day) dry intervals across all seasons. For other species-ecosystem combinations, two richness patterns were observed. Compared to AVG2, AVG1 had lower water-year precipitation yet more C3 species in upland grasslands, creosotebush, and mesquite shrublands, and more C4 perennial grasses in tarbush shrublands. AVG1 also had larger amounts of rainfall and more large storms in fall and spring with higher mean depths of storm and lower mean dry-day interval compared with AVG2. While DRY1 and DRY2 had the same amount of precipitation, DRY2 had more C4 species than DRY1 in creosote bush shrublands, and DRY1 had more C3 species than DRY2 in upland grasslands. Most differences in rainfall between these periods occurred in the summer. Legacy effects were observed for C3 species in upland grasslands where no significant change in richness occurred from DRY1 to WET compared with a 41% loss of species from the WET to DRY2 period. The opposite asymmetry pattern was found for C4 subdominant species in creosote bush and mesquite shrublands, where an increase in richness occurred from DRY1 to WET followed by no change in richness from WET to DRY2. Our results show that understanding plant biodiversity of Chihuahuan Desert landscapes as precipitation continues to change will require daily and seasonal metrics of rainfall within a wet-dry period paradigm, as well as a consideration of species traits (photosynthetic pathways, lifespan, morphologies). Understanding these relationships can provide insights into predicting species-level dynamics in drylands under a changing climate.

Increasing Summer Rainfall and Asymmetrical Diurnal and Seasonal Warming Enhanced Vegetation Greenness in Temperate Deciduous Forests and Grasslands of Northern China

Temperate forests and grasslands carry key ecosystem functions and provide essential services. Remote-sensing derived greenness has been widely used to assess the response of ecosystem function to climate and land-cover changes. Although reforestation and grassland restoration have been proposed to enhance the regional greenness in Northern China, the independent contribution of climate without the interference of land-cover change at meso and large scales has rarely been explored. To separate the impacts of climate change on vegetation greenness from those of land-cover/use change, we identified large patches of forests and grasslands in Northern China without land-cover/use changes in 2001–2015 and derived their greenness using MODIS enhanced vegetation index (EVI). We found that most deciduous-broadleaved forest patches showed greening, and the significant slope of the annual mean and maximum EVI are 3.97 ± 0.062 × 10−3 and 4.8 ± 0.116 × 10−3 yr−1, respectively. On the contrary, grassland patches showed great spatial heterogeneity and only those in the east showed greening. The partial correlation analysis between EVI and climate showed that the greening of grassland patches is primarily supported by the increased growing-season precipitation with mean significant coefficient of 0.72 ± 0.01. While wet-year (0.57 ± 0.01) and nongrowing-season precipitation (0.68 ± 0.01) significantly benefit greening of deciduous-broadleaved forests, the altered temperature seasonality modulates their greening spatial-heterogeneously. The increased growing-season minimum temperature might lengthen the growing season and contribute to the greening for the temperature-limited north as shown by positive partial correlation coefficient of 0.66 ± 0.01, but might elevate respiration and reduce greening of the forests in the south as shown by negative coefficient of −0.70 ± 0.01. Daytime warming in growing season is found to favor the drought-tolerant oak dominated forest in the south due to enhanced photosynthesis, but may not favor the forests dominated by less-drought-tolerant birch in the north due to potential water stress. Therefore, grassland greening was essentially promoted by the growing-season precipitation, however, in addition to being driven by precipitation, greening of deciduous forests was regulated spatial-heterogeneously by asymmetrical diurnal and seasonal warming which could be attributed to species composition.

Plastic Film Mulching Sustains High Maize (Zea mays L.) Grain Yield and Maintains Soil Water Balance in Semiarid Environment

Ridge–furrow cultivation with plastic film mulching has been widely used for many years to increase crop yields in semiarid regions. The long-term effects of plastic mulching on crop yield and soil water balance need to be seriously considered to assess the sustainability of this widely used field management technique. A seven-year maize field experiment was conducted during 2012–2018 to estimate the yield sustainability and soil water balance with two treatments—mulching (yes; no) and nitrogen fertilization (yes; no). This resulted in the following four groups—no film mulching, no N application (M0N0); film mulching, no N application (M1N0); no film mulching, N application (M0N1); film mulching and N application (M1N1). Our results show that plastic mulching significantly increased maize yield. A combination of mulching and nitrogen application had the highest sustainability yield index (SYI) of 0.75, which was higher than the other three treatments, with SYI values of 0.31, 0.33, and 0.39, respectively. Plastic film mulching increased soil water content and water storage in both the sowing and harvesting periods and did not cause the formation of dry soil layers. Precipitation storage efficiency (PSE) in the nongrowing season played a key role in maintaining the soil water balance and it was positively affected by plastic film mulching. Our research indicates that plastic mulching and N application could maintain maize yield sustainability and the soil water balance of agriculture in semiarid regions. In addition, we highlight the importance of nongrowing season precipitation, and thus, we suggest that mulching the field land with plastic film throughout the whole year should be adopted by farmers to store more precipitation, which is important to crop growth.

Climatic and management determinants of large herbivore production in semiarid grassland

Knowledge of climatic and management influences on large herbivore production (LHP, kg ha−1) is needed for low productivity, semiarid grasslands to address potential consequences of both increasing climate variability and the need to increase animal protein for human consumption. Here, we evaluate the influence of climatic variability and herbivore density on LHP in semiarid grassland using a unique long-term (80 years: 1939–2018) grazing study with three grazing intensities based on forage utilization (light, moderate and heavy). Seasonal variation in precipitation, but not temperature, was the primary influence on LHP. Winter (October-March) and spring (April-June), but not summer (July-September), precipitation during the current year positively influenced LHP across the 3 grazing intensities, whereas prior growing season (prior April-September) precipitation was consistently a negative influence. Although spring precipitation was the most influential seasonal weather variable for LHP, the effect of winter precipitation closely followed under all three grazing intensities, suggesting that non-growing season precipitation is essential for soil water storage to initiate production of sufficient high-quality forage in the subsequent grazing season, resulting in a positive feedback on LHP. A key finding from our analysis was that the effect of summer precipitation is smaller than the combined effects of winter and spring precipitation. As such, much of the variation in LHP can be predicted by seasonal weather parameters that are known early in the growing season. The magnitude of seasonal precipitation effects on LHP was greatest for heavy grazing; consequently LHP with heavy grazing is more reliant on primary production produced in the current year to increase LHP as forage quantity is more limiting than forage quality. Moreover, stability of LHP across years (range: 7.5 to 34.6 kg ha−1) was less with heavy grazing, which results in “boom-bust” economics that threaten sustainability of operations. Management adaptations to mitigate climatic variability, therefore, will be most necessary and advantageous when land managers employ heavy grazing intensities. Despite the substantial interannual variability in precipitation that characterizes semiarid grasslands, our results show that proactive flexibility by land managers in adjusting grazing management decisions to seasonal precipitation amounts forecasted for the winter and spring seasons would reduce enterprise risk and improve confidence in decision-making, profitability, production efficiency and environmental sustainability from semiarid grasslands.

Base cation concentrations in forest litter and topsoil have different responses to climate and tree species along elevational gradients

The forest litter is an essential reservoir of nutrients in forests, supplying a large part of absorbable base cations (BC) to topsoil, and facilitating plant growth within litter-soil system. To characterize elevational patterns of base cation concentrations in the forest litter and topsoil, and explore the effects of climate and tree species, we measured microclimate and collected the forest litter and topsoil (0-10 cm) samples across an elevational range of more than 2000 m (1243 ~ 3316 m a.s.l.), and analyzed the concentrations of BC in laboratory. Results showed that: 1) litter Ca concentration displayed a hump-shaped pattern along the elevational gradients, but litter K and Mg showed saddle-shaped patterns. Soil Ca concentration increased with elevation, while soil K and Mg had no significant changes. 2) Ca concentration in the forest litter under aspen (Populus davidiana) was significantly higher than that in all other species, but in topsoil, Ca concentration was higher under coniferous larch and fir (Larix chinensis and Abies fargesii). Litter K and Mg concentrations was higher under coniferous larch and fir, whereas there were no significant differences among tree species in the concentrations of K and Mg in topsoil. 3) Climatic factors including mean annual temperature (MAT), growing season precipitation (GSP) and non-growing season precipitation (NGSP) determined BC concentrations in the forest litter and topsoil. Soil C/N and C/P also influenced BC cycling between litter and soil. Observation along elevations within different tree species implies that above-ground tree species can redistribute below-ground cations, and this process is profoundly impacted by climate. Litter and soil Ca, K and Mg with different responses to environmental variables depend on their soluble capacity and mobile ability.

Seasonal and interannual variations of ecosystem photosynthetic features in an alpine dwarf shrubland on the Qinghai-Tibetan Plateau, China

Ecosystem photosynthetic characteristics are of utmost importance for the estimation of regional carbon budget, but such characteristics are not well understood in alpine regions. We collected CO2 flux data measured by eddy covariance technique over an alpine dwarf shrubland on the Qinghai-Tibetan Plateau during years 2003-2010; and we quantified the temporal patterns of ecosystem apparent quantum yield (a), saturated photosynthetic rate (P max), and ecosystem dark respiration (R De). Results showed that the strong seasonality of a and R De was driven mainly by air temperature (T a), whereas that of P max was much more determined by leaf area index rather than abiotic factors. Diurnal thermal fluctuation inhibited significantly the daytime photosynthetic capacity. Stepwise regression revealed that the seasonal deviations of a, P max, and R De were significantly controlled by T a. The annual a was regulated mainly by annual growing season T a, which indicated that the response of ecosystem a was instant. The annual variations of P max correlated positively with soil temperature 5 cm below ground (T s) of the annual nongrowing season and those of R De related negatively with the annual nongrowing season precipitation. We suggested that a lagged response regulated the annual P max and the annual R De. Annual deviations of a and R De were both significantly controlled by annual T s, and those of P max were marginally determined by annual PPFD. Thus, the future warming scenario, especially significant for nongrowing seasonal warming in the Qinghai-Tibetan Plateau, would favor ecosystem photosynthetic capacity in the alpine dwarf shrubland.

Assessing the impact of climate variability on catchment water balance and vegetation cover

Abstract. Understanding the interactions among climate, vegetation cover and the water cycle lies at the heart of the study of watershed ecohydrology. Recently, considerable attention is being paid to the effect of climate variability on catchment water balance and also associated vegetation cover. In this paper, we investigate the general pattern of long-term water balance and vegetation cover (as reflected by fPAR) among 193 study catchments in Australia through statistical analysis. We then employ the elasticity analysis approach for quantifying the effects of climate variability on hydrologic partitioning (including total, surface and subsurface runoff) and on vegetation cover (including total, woody and non-woody vegetation cover). Based on the results of statistical analysis, we conclude that annual runoff (R), evapotranspiration (E) and runoff coefficient (R/P) increase with vegetation cover for catchments in which woody vegetation is dominant and annual precipitation is relatively high. Control of water available on annual evapotranspiration in non-woody dominated catchments is relatively stronger compared to woody dominated ones. The ratio of subsurface runoff to total runoff (Rg/R) also increases with woody vegetation cover. Through the elasticity analysis of catchment runoff, it is shown that precipitation (P) in current year is the most important factor affecting the change in annual total runoff (R), surface runoff (Rs) and subsurface runoff (Rg). The significance of other controlling factors is in the order of annual precipitation in previous years (P−1 and P−2), which represents the net effect of soil moisture and annual mean temperature (T) in current year. Change of P by +1% causes a +3.35% change of R, a +3.47% change of Rs and a +2.89% change of Rg, on average. Results of elasticity analysis on the maximum monthly vegetation cover indicate that incoming shortwave radiation during the growing season (Rsd,grow) is the most important factor affecting the change in vegetation cover. Change of Rsd,grow by +1% produces a −1.08% change of total vegetation cover (Ft) on average. The significance of other causative factors is in the order of precipitation during growing season, mean temperature during growing season and precipitation during non-growing season. Growing season precipitation is more significant than non-growing season precipitation to non-woody vegetation cover, but both have equivalent effects to woody vegetation cover.

Corn Yield Response to Deficit Irrigation

Because dwindling water supplies are limiting crop production, a field study was conducted during 2005-2009 in southwest Kansas to determine the yield response of corn to irrigation and evapotranspiration (ETc), and to document plant growth parameters and soil water use. Corn was grown in a five-year rotation of corn-corn-wheat-grain sorghum-sunflower. Results from the corn after sunflower and corn after corn are presented here. Six irrigation treatments were produced by applying 25 mm of irrigation every 5 to 17 days. Year-to-year grain yields averaged over irrigation and crop sequence appeared to be correlated with leaf area index, which possibly reflected the severity of hail events that occurred in four of the five years of the study. However, dry matter accumulation per plant did not vary across irrigation treatments. Surface residue coverage from the previous year's crop was 38% for sunflower and 61% for corn. ETc and productivity, also known as water use efficiency (WUE), decreased significantly as irrigation decreased. The deficit irrigation treatments used more of the previous non-growing season precipitation than the fully irrigated treatment due to greater soil water storage capacity in the drier soil profile. Furthermore, these treatments extracted more soil water during the growing season as irrigation decreased. Linear models of ETc predicted grain and dry matter yields with R2 values of 0.67 and 0.59, respectively. The relationship of relative grain yield and ETc was also linear and more pronounced, with an R2 value of 0.82. In contrast, the relationship of relative yield and irrigation followed a curvilinear model. During the five-year study, variability in yields increased as irrigation decreased, illustrating a greater income risk with less irrigation. Yield response to irrigation, especially over multiple years, is essential information to build economic studies of cropping alternatives, deficit irrigation management, and income risk. These relationships need to be developed regionally to characterize the effects of environmental factors, especially precipitation.

Soil water recharge in a semi-arid temperate climate of the Central U.S. Great Plains

Abstract The amount of soil water at the beginning of the growing season has a large impact on crop yields in rainfed agriculture, especially in semi-arid regions and in years with below-average rainfall in more humid climates. Robust algorithms are needed to estimate soil water storage before planting to aid crop management decisions. The main objectives of this paper are to investigate soil water recharge during the non-growing season (October 20 to May 1) in a semi-arid, temperate ecosystem in south-central Nebraska (USA) and to evaluate empirical models to estimate soil water content at the beginning of the summer-crop growing season. A database of soil water content measurements collected over 5 years at nine locations in south-central Nebraska was used to estimate available water-holding limits in the soil profile and to determine the change in available soil water during the non-growing season. Regression analysis was performed to analyze the relationship among soil water recharge, residual soil water (i.e., soil water content at the end of the previous growing season), total precipitation, and available water-holding capacity (AWHC) in the root zone to 1.5 m. Precipitation storage efficiency (PSE) was calculated as the quotient of soil water recharge and total non-growing season precipitation. Predictive models to estimate soil water content at the beginning of summer-crop growing season were derived from these analyses. A large portion of the variation in soil water recharge was explained by residual soil water and precipitation. PSE averaged 28% across site-years; low PSE values were associated with high residual soil water and/or low AWHC. Two predictive models (linear and linear-plateau) that used residual soil water, total precipitation, and AWHC as independent variables explained 75–80% of the variation in the measured soil water content at the beginning of the summer-crop growing season. These empirical models represent a new tool to estimate soil water content by planting date of summer crops. Site-management conditions such as residue amount and its architecture, tillage system, soil texture, and terrain slope are not currently accounted for in these models and would likely improve predictive capacity.

Growth response of ponderosa pine (Pinus ponderosa) to climate in the eastern Cascade Mountains, Washington, U.S.A.: Implications for climatic change1

Climatic change is expected to cause dramatic shifts in low-elevation treeline in mountainous environments. Ponderosa pine (Pinus ponderosa) were sampled across an elevation gradient adjacent to the Methow Valley of the Okanogan National Forest in the eastern Cascades, Washington to examine the potential response of ponderosa pine to climatic change. Response function analyses were used to compare climate-growth relationships among 12 sites, four elevations on three different mountains. Response function analysis attributes 42-55% of the inter-annual variation in growth to climate. Growth is positively correlated with November precipitation prior to the growing season on all 12 sites, suggesting that November precipitation is critical for increased root growth, increased nutrient availability through decomposition, building snowpack, or non-growing season photosynthesis and carbon storage. Growth is positively correlated with previous October, January, June, and July precipitation at more than one site. Temperature is not correlated with growth on any sites. Climate models predict that the Pacific Northwest will experience warmer and wetter winters and drier summers in the future. Growth-climate correlations suggest that the short-term growth response of ponderosa pine is most sensitive to non-growing season precipitation. Therefore, predicting ponderosa pine’s response to projected climatic change is problematic, with wetter falls increasing growth and drier summers decreasing growth. Our results indicate that ponderosa pine is much more sensitive to precipitation than temperature and that any predictions of this arid species’ response to climatic change are difficult, due to uncertainty in predicting future precipitation patterns.

Factors Associated with Global Occurrences of Septoria nodorum Blotch and Septoria tritici Blotch of Wheat

Changes in incidence and severity of Septoria nodorum blotch (SNB) and Septoria tritici blotch (STB) have been noted in recent years in several wheat-growing areas of the world. A questionnaire was designed to identify factors associated with occurrence and development of these diseases. The questionnaire was sent to Septoria workers worldwide, and 71 responses were received and analyzed. The most important factors found relating to disease occurrence were: 1) latitude, 2) non-growing season precipitation, 3) growing season precipitation, 4) application of phosphorus, and 5) frequency of minimum or reduced tillage

Ground‐Water Nitrate Pollution in Rural Areas

ABSTRACTIn humid regions, chemical pollutants such as nitrate readily enter surficial aquifers as slugs during nongrowing season precipitation recharge periods and move slowly through the aquifer to nearby drainage tiles, streams, or pumping wells. Little mixing, with native ground water seems to occur, and for this reason, fairly small quantities of pollution in bulb form may enter a well as a high‐concentration slug long after its initial entry into the aquifer. Nitrate slug concentrations in excess of 6000 ppm have been delineated in one surficial aquifer in Illinois.Recent ground‐water pollution studies indicate that trees and plants may temporarily store large quantities of nutrients during the growing season for release to aquifers during subsequent nongrowing seasons. Considering this, it appears imperative that farm fertilizer application and human or animal waste disposal on farmlands be limited only to the growing season of each year and only in quantities that will not ultimately cause ground‐water nitrate concentrations to rise above toxic levels.