Soil Solution NO3 Research Articles

Soil solution chemistry weak response to long-term N addition points towards a strong resilience of northeastern American forests to past and future N deposition

Northern temperate and boreal forests are large biomes playing crucial ecological and environmental roles, such as carbon sequestration. Despite being generally remote, these forests were exposed to anthropogenic nitrogen (N) deposition over the last two centuries and may still experience elevated N deposition as human activities expand towards high latitudes. However, the impacts of long-term high N deposition on these N-limited forest ecosystems remain unclear. For 18 years, we simulated N deposition by chronically adding ammonium nitrate at rates of 3 (LN treatment) and 10 (HN treatment) times the ambient N deposition estimated at the beginning of the experiment at a temperate sugar maple and a boreal balsam fir forest site, both located in northeastern America. LN and HN treatments corresponded respectively to addition of 26 kgN·ha−1·yr−1 and 85 kgN·ha−1·yr−1 at the temperate site and 17 kgN·ha−1·yr−1 and 57 kgN·ha−1·yr−1 at the boreal site. Between 2002 and 2018, soil solution was collected weekly during summer and concentrations of NO3−, NH4+, Ca2+ and pH were measured, totalling ~12,700–13,500 observations per variable on the study period. N treatments caused soil solution NO3−, NH4+ and Ca2+ concentrations to increase while reducing its pH. However, ion responses manifested through punctual high concentration events (predominantly on the HN plots) that were very rare and leached N quantity was extremely low at both sites. Therefore, N addition corresponding to 54 years (LN treatment) and 180 years (HN treatment) of accelerated ambient N deposition had overall small impacts on soil solution chemistry. Our results indicate an important N retention of northeastern American forests and an unexpected strong resilience of their soil solution chemistry to long-term simulated N deposition, potentially explained by the widespread N-limitation in high latitude ecosystems. This finding can help predict the future productivity of N-limited forests and improve forest management strategies in northeastern America.

Nitrate leaching and its susceptibility in response to elevated nitrogen deposition in Japanese forests

ABSTRACT Understanding the variability in nitrogen (N) leaching from forested ecosystems and its susceptibility to elevated atmospheric N deposition is essential for assessing the impact of atmospheric N deposition on forested ecosystems. This study explored the site variability in nitrate (NO3 −) leaching from forested areas and its susceptibility to elevated atmospheric N deposition from five sites in broadleaved forests grown on Andosols and Cambisols from northern to southern Japan. Site variability of NO3 − leaching and its possible contributing factors, including soil parent material and net nitrification rate, were evaluated using a generalized linear model with Akaike’s Information Criterion used for model selection. Three-year experimental N fertilization (50 kg N ha−1 yr−1 as ammonium nitrate) experiments were conducted at each site to evaluate the susceptibility of NO3 − leaching to elevated atmospheric N deposition. Soil solution NO3 − concentration below the rooting zone (50 cm below the ground surface) was variable among the five sites and was the highest in Shibecha, one of the Andosols sites in northern Japan where soil net nitrification tended to be higher than elsewhere. In model selection for NO3 − leaching, the soil parent material was not selected but soil net nitrification was selected, indicating that soil net nitrification could explain the variability of NO3 − leaching. A significant increase in NO3 – leaching after the 3-year N fertilization experiment was observed at the site with the highest levels of NO3 – leaching (Shibecha), indicating that the susceptibility of NO3 – leaching is high in locations where N leaching was originally high.

Reductions in soil water nitrate beneath a perennial grain crop compared to an annual crop rotation on sandy soil

Nitrate (NO3--N) leaching into groundwater as a result of high nitrogen (N) fertilizer rates to annual crops presents human health risks and high costs associated with water treatment. Leaching is a particularly serious concern on sandy soils overlying porous bedrock. Intermediate wheatgrass (IWG) [Thinopyrum intermedium (Host.) Barkw. & D.R. Dewey], is a perennial grass that is being bred to produce agronomically and economically viable grain, which is commercially available as Kernza®. Intermediate wheatgrass is a low-input crop has the potential to produce profitable grain and biomass yields while reducing NO3--N leaching on sandy soils compared with common annual row crop rotations in the Upper Midwest. We compared grain yields, biomass yields, soil solution NO3--N concentration, soil extractable NO3--N, soil water content, and root biomass under IWG and a conventionally managed corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] rotation for 3 years on a Verndale sandy loam in Central Minnesota. Mean soil solution NO3--N was 77–96% lower under IWG than the annual crop rotation. Soil water content was greater under annuals compared to IWG early in the growing season, suggesting greater water use by IWG during this time. Interactions between crop treatments and depth were observed for soil water content in Year 3. Root biomass from 0 to 60 cm below the soil surface was five times greater beneath IWG compared to soybean, which may explain differences in soil extractable and solution NO3--N among crops. With irrigation on coarse structured soils, IWG grain yields were 854, 434, and 222 kg ha−1 for Years 1–3 and vegetative biomass averaged 4.65 Mg ha−1 yr−1; comparable to other reports on heavier soils in the region. Annual crop grain yields were consistent with local averages. These results confirm that IWG effectively reduces soil solution NO3--N concentrations even on sandy soils, supporting its potential for broader adoption on land vulnerable to NO3--N leaching.

Continuous in situ soil nitrate sensors: The importance of high‐resolution measurements across time and a comparison with salt extraction‐based methods

AbstractSoil NO3– affects microbial processes, plant productivity, and environmental N losses. However, the ability to measure soil NO3– is limited by labor‐intensive sampling and laboratory analyses. Hence, temporal variation in soil solution NO3– concentration is poorly understood. We evaluated a new potentiometric sensor that continuously measures soil solution NO3– concentration with unprecedented specificity due to a novel membrane that serves as a barrier to interfering anions. First, we compared sensor and salt extraction‐based measurements of soil NO3– in well‐controlled laboratory conditions. Second, using 60 d of in situ soil NO3– measurements every 10 s, we quantified temporal variation and the effect of sampling frequency on field estimations of mean daily NO3– concentration both within and across days. In the laboratory, sensors measured soil NO3– concentration without significant difference from theoretical adjusted soil NO3– concentration or conventional salt extraction‐based methods. In the field, the sensors demonstrated no within‐day pattern in soil NO3– concentration, although individual measurements within a day differed by as much as 20% from the daily mean. Across days, when soil solution NO3– was dynamic (early spring) and sampling frequency was >5 d, estimates of mean daily NO3– concentration were >20% from the actual mean daily concentration. In situ soil sensors offer potential to improve fundamental and applied sciences. However, in most situations, sensors will measure soil properties in a different manner than conventional salt‐extract soil sampling‐based approaches. Research will be required to interpret sensor measurements and optimize sensor deployment.

氮添加对亚热带常绿阔叶林土壤溶液化学特性的影响

土壤溶液被称作"土壤的血液"，是土壤中各种生物化学反应的中介物质，在外界环境发生变化时，土壤溶液化学成分能在其他土壤指标尚无变化之前对环境变化做出迅速响应。为了探索持续增加的大气氮（N）沉降对森林生态系统的影响，以华西雨屏区亚热带常绿阔叶林为对象，设置对照（CK，0 g m^-2 a^-1），低N（LN，5 g m^-2 a^-1），高N（HN，15 g m^-2 a^-1）三种N处理，通过人工施加硝酸铵（2017年9月起改施硝酸钠及氯化铵）的方法模拟N沉降增加情景，N处理42个月后，使用负压土壤溶液采样器定位收集A层（37-45 cm）及B层（52-60 cm）土壤溶液，并进行分析（每月1次，为期1 a）。结果表明：对照处理中A、B两层土壤溶液NO₃^-浓度达（3.94±0.77）mg/L、（4.27±1.13）mg/L，N添加显著提高两层土壤溶液NO₃^-浓度和B层NH₄⁺浓度；N添加显著降低土壤溶液pH，且显著增加Al³⁺浓度，Ca²⁺和Mg²⁺含量有增加趋势，但影响不显著；N处理使A层土壤溶液可溶性有机碳（DOC）浓度显著降低，对两层土壤溶液芳香化指数（AI）无显著影响；两层土壤溶液电导率（EC）及氧化还原电位（Eh）显著增加；此外，两层土壤溶液中许多化学成分均呈现极显著相关，特别是NO₃^-与EC相关系数达到了0.855。本研究中，对照处理极高的NO₃^-含量以及B层土壤溶液硝酸盐浓度高于A层，表明该亚热带常绿阔叶林N的有效性超过植物和微生物的总营养需求而发生淋溶，该生态系统已达到氮饱和状态，此外N添加会显著促进土壤酸化和铝离子活化，表层土壤溶液DOC的降低一定程度反映了凋落物分解受N添加的抑制作用，显著升高的氧化还原电位可能导致土壤中某些金属元素的迁移率降低。;Soil solution is called soil blood, which is the medium of various biochemical reactions in the soil. When the external environment changes, the chemical composition of the soil solution can quickly respond to the environmental changes before other soil indicators have changed. In order to explore the impact of increasing atmospheric nitrogen (N) deposition on forest ecosystems, we set up three N treatments including the control (CK, 0 g m^-2 a^-1), low N (LN, 5 g m^-2 a^-1), and high N (HN, 15 g m^-2 a^-1) in a subtropical evergreen broad-leaved forest in the raining area of west China, artificially applied ammonium nitrate to simulate the increase of N deposition (replaced with sodium nitrate and ammonium chloride since September 2017). After 42 months of N treatment, soil solution from two soil layers (layer A:37-45 cm, layer B:52-60 cm) was collected using negative pressure soil solution sampler and analyzed chemical composition once a month for a year. In CK, soil solution NO₃^- concentration reached (3.94±0.77) mg/L(layer A) and (4.27±1.13) mg/L(layer B). Nitrogen additions significantly increased soil solution NO₃^- and NH₄⁺ concentration in layer B. Nitrogen additions significantly reduced soil solution pH value and increased Al³⁺ concentration, and the concent of Ca²⁺ and Mg²⁺ increased with no significant in statistics. Nitrogen treatment significantly reduced the dissolved organic carbon (DOC) concentration in layer A, and had no significant effect on the aromatic index (AI) of soil solution in the two layers. The electrical conductivity (EC) and the redox potential (Eh) of soil solution increased significantly in two soil layers. In addition, many chemical components in the soil solution of two layers had extremely significant correlation, especially the correlation coefficient between NO₃^- and EC reached 0.855. In this study, the extremely high NO₃^- concentration in CK and higher NO₃^- concentration in layer B than in layer A indicated that the nitrogen availability of the subtropical evergreen broad-leaved forest exceeded the total nutrient requirements of plants and microorganisms, and the ecosystem had reached the state of nitrogen saturation. In addition, nitrogen addition significantly promoted soil acidification and aluminum activation. The decrease of DOC in surface soil solution to some extent reflected the inhibition of litter decomposition by nitrogen addition. The significant increase of redox potential may lead to the decrease of the mobility of some metal elements in the soil.

Immediate Effects of Prescribed Fire on Sub-Surface Water Quality in a Managed Yellow Pine Forest

Although prescribed fire is a forest management tool annually applied to nearly one million hectares across the southeastern United States, little is known about how prescribed fire influences soil water quality in the region. Sub-surface pools of nitrogen (N) and phosphorous (P) are important indicators of ecosystem response to disturbance and are likely modified, at least temporarily, by fire. We undertook a five-month study to examine sub-surface nutrient concentrations and pH in a managed yellow pine (Pinus sp.) forest before and after a low-severity dormant season prescribed fire in the Southern Blue Ridge Mountains of South Carolina. Between February and July 2019, soil solution was collected weekly from a 30 cm porous cup suction lysimeters and analyzed for ammonium (NH4+), nitrate (NO3-), and orthophosphate (PO43-), as well as pH. We compared the five-month mean and maximum nutrient concentrations and pH to quantify the immediate effects of prescribed fire. The prescribed fire caused significant pH buffering towards neutrality in burned transects relative to the control. There was no change in soil solution NO3-. The prescribed fire caused a significant increase in maximum NH4+ (18.0 mg/L) and PO43- (6.6 mg/L) concentrations. Post-fire NH4+ concentrations reached a maximum of 18.0 mg/L before declining three weeks post-fire. PO43- concentrations in burned stands reached a maximum of 6.6 mg/L and remained elevated for four weeks post-fire. Nutrient leaching was minimal due to complexion to soil cations and ground water uptake from regenerating vegetation at the onset of the growing season. The PO43- and NH4+ pulses in this study are likely influenced by fire-induced changes to soil chemistry and future studies should examine the homogeneity of pH buffering and nutrient pulses across the burned landscape.

Nitrification represents the bottle-neck of sheep urine patch N2O emissions from extensively grazed organic soils

Extensively grazed grasslands are understudied in terms of their contribution to greenhouse gas (GHG) emissions from livestock production. Mountains, moorlands and heath occupy 18% of the UK land area, however, in situ studies providing high frequency N2O emissions from sheep urine deposited to such areas are lacking. Organic soils typical of these regions may provide substrates for denitrification-related N2O emissions, however, acidic and anoxic conditions may inhibit nitrification (and associated emissions from nitrification and denitrification). We hypothesised urine N2O-N emission factors (EFs) would be lower than the UK country-specific and IPCC default value for urine, which is based on lowland measurements. Using automated GHG sampling chambers, N2O emissions were determined from real sheep urine (930 kg N ha−1) and artificial urine (920 kg N ha−1) applied in summer, and from an artificial urine treatment (1120 kg N ha−1) and a combined NO3− and glucose treatment (106 kg N ha−1; 213 kg C ha−1) in autumn. The latter treatment provided an assessment of the soils capacity for denitrification under non-substrate limiting conditions. The artificial urine-N2O EF was 0.01 ± 0.00% of the N applied in summer and 0.00 ± 0.00% of the N applied in autumn. The N2O EF for real sheep urine applied in summer was 0.01 ± 0.02%. A higher flux was observed in only one replicate of the real urine treatment, relating to one chamber where an increase in soil solution NO3− was observed. No lag phase in N2O emission was evident following application of the NO3− and glucose treatment, which emitted 0.69 ± 0.15% of the N applied. This indicates nitrification rates are the bottle-neck for N2O emissions in upland organic soils. We calculated the potential impact of using hill-grazing specific urine N2O EFs on the UK inventory of N2O emissions from sheep excreta, and found a reduction of ca. 43% in comparison to the use of a country-specific excretal EF.

Herbaceous perennial biomass production on frequently saturated marginal soils: Influence on N2O emissions and shallow groundwater

Warm season perennial grasses grown for biomass have been suggested as alternative cropping systems on marginal soils to increase farm profit, reduce nitrous oxide (N2O) emissions, and improve water quality. The objectives of this study were to determine: 1) how warm season perennial grasses, switchgrass (Panicum virgatum) and Miscanthus (Miscanthus x giganteus), compare to cool season grasses as streamside buffers on poorly drained marginal soils 2) if inorganic or organic nutrient additions improve biomass yield and affect environmental outcomes? 3) which soil variables influence N2O emissions in situ? We measured soil N2O emissions, soil solution nitrate (NO3−), ammonium (NH4+), O2, moisture, and temperature, along with shallow groundwater NH4+, NO3−, and ortho-phosphate during two growing seasons (2012–2013). N2O emissions were similar across unfertilized warm season grasses and cool season grasses. However, when switchgrass was fertilized with ammonium sulfate or broiler manure, N2O emissions increased significantly. N2O emissions were weakly correlated with soil solution NO3− concentrations and water filled pore space. Shallow groundwater N was elevated under switchgrass fertilized with ammonium sulfate, broiler manure, and when grown with the legume (Desmodium canadense) when compared to unfertilized switchgrass, Miscanthus, and cool season grasses. In 2013 dry aboveground biomass production did not differ among switchgrass treatments which averaged 10 Mg ha. Biomass production was significantly higher for Miscanthus (18.5 Mg ha). The results indicate that unfertilized switchgrass and Miscanthus are as effective as cool season grasses at mitigating N2O emissions and improving water quality, and that Miscanthus has potential production advantages over switchgrass grown on frequently saturated soils.

Variability, drivers, and effects of atmospheric nitrogen inputs across an urban area: Emerging patterns among human activities, the atmosphere, and soils

Atmospheric deposition of nitrogen (N) is a major input of N to the biosphere and is elevated beyond preindustrial levels throughout many ecosystems. Deposition monitoring networks in the United States generally avoid urban areas in order to capture regional patterns of N deposition, and studies measuring N deposition in cities usually include only one or two urban sites in an urban-rural comparison or as an anchor along an urban-to-rural gradient. Describing patterns and drivers of atmospheric N inputs is crucial for understanding the effects of N deposition; however, little is known about the variability and drivers of atmospheric N inputs or their effects on soil biogeochemistry within urban ecosystems. We measured rates of canopy throughfall N as a measure of atmospheric N inputs, as well as soil net N mineralization and nitrification, soil solution N, and soil respiration at 15 sites across the greater Boston, Massachusetts area. Rates of throughfall N are 8.70±0.68kgNha−1yr−1, vary 3.5-fold across sites, and are positively correlated with rates of local vehicle N emissions. Ammonium (NH4+) composes 69.9±2.2% of inorganic throughfall N inputs and is highest in late spring, suggesting a contribution from local fertilizer inputs. Soil solution NO3− is positively correlated with throughfall NO3− inputs. In contrast, soil solution NH4+, net N mineralization, nitrification, and soil respiration are not correlated with rates of throughfall N inputs. Rather, these processes are correlated with soil properties such as soil organic matter. Our results demonstrate high variability in rates of urban throughfall N inputs, correlation of throughfall N inputs with local vehicle N emissions, and a decoupling of urban soil biogeochemistry and throughfall N inputs.

Nitrate and dissolved organic carbon mobilization in response to soil freezing variability

Reduced snowpack and associated increases in soil freezing severity resulting from winter climate change have the potential to disrupt carbon (C) and nitrogen (N) cycling in soils. We used a natural winter climate gradient based on elevation and aspect in a northern hardwood forest to examine the effects of variability in soil freezing depth, duration, and frequency on the mobilization of dissolved organic carbon (DOC) and nitrate (NO3 −) in soils over the course of 2 years. During a winter with a relatively thin snowpack, soils at lower elevation sites experienced greater freezing and especially variable freeze/thaw cycles, which in turn led to greater leaching of DOC from the organic horizon during the following growing season. In contrast to several previous field manipulation studies, we did not find changes in soil solution NO3 − concentrations related to soil freezing variables. Our results are consistent with a soil matrix disturbance from freezing and thawing which increases leachable C. These results build upon previous laboratory experiments and field manipulations that found differing responses of DOC and NO3 − following soil freezing, suggesting that mobilization of labile C may suppress NO3 − losses through microbial immobilization of N. This research highlights the importance of studying natural variation in winter climate and soil freezing and how they impact soil C and N retention, with implications for surface water runoff quality.

Laboratory Investigation of Boundary Condition Impacts on Nitrate Anion Exclusion in an Unsaturated Soil

Transient unsaturated horizontal column experiments were conducted with a loam soil, under variable boundary conditions, to obtain added insight on nitrate (NO3−) anion exclusion processes. The boundary conditions evaluated were column inlet soil water content (θ0), initial soil water content (θi), column inlet soil solution NO3−–N concentration (C0), and the initial soil solution NO3− concentration (Ci). Results consistent with significant anion exclusion of NO3− were exhibited in all experiments. For tests with NO3− solution injected at the column inlet, the soil solution NO3− concentration at the inlet was found to be 12 to 19% less than the injected NO3− concentration. For these same tests, where initial soil conditions were dry with no NO3− originally present, the NO3− concentrations at the wetting front edge were substantially greater than the injected NO3− concentration, while for initially wetter conditions, with or without NO3− originally present, the NO3− front was found to extend well beyond the theoretical “piston front” separating the injected solution from displaced initial soil water. For displacement tests, in which water was injected at the inlet of a column initially wetted with a NO3− solution, the soil solution NO3− front also extended beyond the piston front. Inlet NO3− excluded water content ranged from 0.03 to 0.06 and was not affected by pore water velocity, but did have a modest positive correlation with total water content (r = 0.60 and r = 0.67). The findings of this investigation provide a better overall understanding of nitrate movement through the soil profile.

Effect of an agri-environmental measure on nitrate leaching from a beef farming system in Ireland

Agricultural nitrogen (N) management remains a key environmental challenge. Improving N management is a matter of urgency to reduce the serious ecological consequences of the reactive N. Nitrate (NO3−–N) leaching was measured under suckler beef production systems stocked at two intensities: (1) intensive, 210kg organic Nha−1 with two cut silage harvests; and (2) rural environmental protection scheme (REPS), 170kg organic Nha−1 with one cut silage harvest. Three replicate plots of each treatment were instrumented with ceramic cups (8 per plot), randomly placed within each plot at a depth of 1m to collect soil solution for NO3−–N at 50kPa suction to collecting vessels one week prior to sampling. Samples were taken on a total of 53 sampling dates over 3 winter drainage periods (2002/03, 2003/04 and 2004/05). Over the course of the experiment the mean annual soil solution NO3−–N concentration exceeded the MAC twice out of 15 means (5 treatments over 3 years). The REPS grazing and silage sub treatments had significantly lower mean annual soil solution total oxidized N (TON) concentrations than the respective intensive treatments in years 2 and 3. Annual total NO3−–N losses over the three years in intensive and REPS systems ranged from 55 to 71 and 15 to 20kgNha−1, respectively. Mean N surpluses in intensive and REPS systems were 210 and 95kgha−1, respectively with the corresponding mean N inputs of 272 and 124kgNha−1. The reduction in N inputs under the REPS system results in lower N leaching losses and contributed to a significant reduction in pressures on water quality.

Impact of potassium on plant uptake of non-exchangeable NH4 +-N

Release of ‘non-exchangeable’ NH4 +-N from interlayers of 2:1 clay minerals is postulated to depend not only on soil solution NH4 +-N concentration but also on the concentration of K+ and Ca2+. Concentrations of all three cations are altered in rhizosphere compared to soil solution at larger distance from the root surface. Non-exchangeable NH4 +-N pool was labelled with 15 N. Treatments including application of K+, Ca2+ and K+ + Ca2+ were established. In a compartment system approach we analysed changes in soil solution concentrations of 15NH4 +-N, 15NO3 −-N, K+ and Ca2+ in situ at different distances from the root surface over time and related them to the release of non-exchangeable 15NH4 +-N and uptake of 15 N by plants. The 15 N enrichment in plant tissue was significantly lower in treatments with K+ application compared to those without. This was in line with smaller depletion of non-exchangeable 15NH4 +-N in the rhizosphere for these treatments and also with lower 15 N abundance in soil solution NO3 −-N fraction. Hence, K+ application hampered the release of NH4 + from the interlayers. A promoting effect of increasing Ca2+ concentrations on release of non-exchangeable NH4 +-N could not be evaluated since the Ca2+ concentration in soil solution was largely controlled by small amounts of carbonate contained in the substrate and thus the addition of Ca2+ did not result in a relevant increase of soil solution Ca2+ concentration as originally intended. The use of 15 N to follow the fate of non-exchangeable NH4 +-N proved very useful as it provides a higher sensitivity for all measured fractions compared to total N. However, as soil N fractions equilibrate with each other labelling one fraction exclusively is not possible.

Changes in soil nitrogen cycling in a northern temperate forest ecosystem during succession

Nitrogen (N) transformations in forest soils are fundamentally important to plant and microbial N nutrition and the N balance of forest ecosystems, but changes in the patterns and rates of N transformations during forest succession are poorly understood. In order to better understand how soil N cycling changes during ecosystem succession, we analyzed four years of soil N cycling measurements in a 90-year-old secondary forest undergoing dieback of early-successional, dominant canopy trees. We expected that tree mortality would decrease root biomass, leading to increased soil NH4 + availability, and that these changes would prompt fundamental shifts in the N cycle such as the initiation of significant nitrification and increased cycling of oxidized N compounds in gas phase and soil solution. As expected, indices of soil NH4 + and NO3 − availability increased with successional stage (defined as the proportion of dead trees), and were negatively correlated with the amount of fine root biomass. However, the standing amount of fine root biomass was not affected by tree mortality; increased soil NH4 + and NO3 − availability therefore more likely resulted from successional increases in N-mineralization than decreases in root N uptake. Nitrification (as indicated by NO efflux as a proxy) increased due to elevated substrate (NH4 +) availability, and the soil solution NO3 − concentration increased as a result. Soil N2O efflux was not affected by succession, nor was it related to other N cycling parameters. Collectively, these results indicate that recent successional advancement has accelerated soil N cycling and shifted the N economy of this ecosystem towards greater importance of NO3 −.

Ion-Selective Electrode Offers Accurate, Inexpensive Method for Analyzing Soil Solution Nitrate in Remote Regions

Nitrogen (N) applied as fertilizer is subject to runoff and leaching. Nitrate (NO3−-N) concentrations in soil solution reflect the impacts of farm management on N cycling. Some of the most remote regions in the world are undergoing rapid land-use change, yet there are major barriers to conducting research in these locations. Fortunately, new tools such as ion-selective electrodes (ISEs) provide low-cost and simple methods for conducting chemical analysis in the field. Researchers compared soil solution NO3−-N concentrations measured using an ISE with concentrations measured on a colorimeter (cadmium reduction) and found good agreement (r2 = 0.96, P < 0.0001) between the two methods. The ISE is also 66 times less expensive than in-country laboratory analysis or shipping and analyzing the samples in the United States. Thus, the ISE is an excellent tool for rapid analysis of N in soil solution, providing critical information on N dynamics in remote regions.

Winter Nitrate Leaching under Different Tillage and Winter Cover Crop Management Practices

The potential for nitrate (NO3–) to leach is enhanced following cultivation of pastoral land, due to rapid mineralization of labile soil organic matter (SOM). In a 7‐yr field trial in New Zealand, we examined the impacts of tillage intensity [intensive (plowing to 20 cm), minimum, or no‐tillage] and winter cover crops (forage rape) on NO3– leaching following cultivation of permanent (sheep [Ovis aries]‐grazed) pasture to grow arable crops (the rotation included barley [Hordeum vulgare L.], wheat [Triticum aestivum L.], and pea [Pisum sativum L.]). Permanent pasture and permanent fallow (maintained plant‐free using herbicides, i.e., not cultivated or fertilized) treatments were included as controls. Losses of NO3––N were calculated from soil solution NO3––N concentrations (measured in ceramic suction cups installed at 600 mm) and drainage volumes. Cumulative NO3––N leached over 7 yr ranged from 20 to 428 kg N ha– 1, with least N lost under pasture. Residual soil mineral N in autumn accounted for ∼30% of the variability in leaching. Nitrate leaching under arable crops generally increased rapidly as winter rainfall (range 78–352 mm yr– 1) increased. Winter cover crops were effective in reducing NO3––N leaching losses, particularly in drier winters when about 50% less N leached where cover crops were grown. On average, annual leaching was only 10 to 18 kg N ha– 1 in the presence of cover crops. Tillage had relatively little influence on leaching, though use of minimum tillage for autumn cultivation resulted in significantly (P &lt; 0.001) less NO3––N leaching than either intensive or no tillage. Largest leaching losses were recorded in the unfertilized, permanent fallow where there was no plant sink for NO3–N derived from SOM mineralization. Growing a crop during the winter period, combined with good N management practices to minimize pre‐winter soil mineral N, provides the best option to keep NO3– leaching within the acceptable range for arable cropping in New Zealand.

Response of soil nitrogen retention to the interactive effects of soil texture, hydrology, and organic matter

AbstractAdvances in nitrogen (N) saturation and retention theories have focused on soil organic matter (SOM) biogeochemistry in the absence of dynamic soil hydrology. Here we exploit two soil types with contrasting textures that span a hillslope gradient to test hypotheses that suggest N saturation symptoms are regulated by the interactive effects of soil texture, OM, and hydrology on N retention capacity (maximum pool size) and N retention kinetics (N retention rate). Down the hillslope gradient, soil solution nitrate (NO3) concentrations sampled with lysimeters increased, while 15NO3‐N retention decreased. Landscape location (upland, hillslope, and toeslope) and soil type interacted to affect soil solution NO3 concentrations so that the downslope increase in NO3 was greater in sandy versus silty soils. These patterns manifest despite a downslope increase in soil organic carbon (SOC) and C/N ratios. A positive correlation between saturated hydraulic conductivity and soil solution NO3 sampled in zero‐tension lysimeters during precipitation events suggested that high hydraulic conductivity promotes periodic rapid NO3 transport at rates that exceed retention kinetics. The downslope increase in soil solution NO3 in spite of a concomitant increase in SOC and C/N ratios provides an important contrast with previous N saturation research that highlights negative correlations between SOM C/N ratios and NO3 concentrations and suggests NO3 transport along connected hillslope flow paths may overwhelm stoichiometric sinks for inorganic N retention in SOM. Our results reveal important gaps in N retention theory based on SOM biogeochemistry alone and demonstrate how coupled biogeochemical and hydrological models can improve predictions of N saturation, particularly when considering periodic advective NO3 transport in the vadose zone. We show that in coarse‐textured soils, low capacity for protection of SOM N by association with fine mineral particles interacts with rapid hydrological flushing of NO3 to enhance the expression of ecosystem N saturation symptoms.

Nitrate in watersheds: Straight from soils to streams?

AbstractHuman activities are rapidly increasing the global supply of reactive N and substantially altering the structure and hydrologic connectivity of managed ecosystems. There is long‐standing recognition that N must be removed along hydrologic flow paths from uplands to streams, yet it has proven difficult to assess the generality of this removal across ecosystem types and whether these patterns are influenced by land use change. To assess how well upland nitrate (NO3−) loss is reflected in stream export, we gathered information from &gt;50 watershed biogeochemical studies that reported nitrate concentrations ([NO3−]) for stream water and for either upslope soil solution or groundwater NO3− to examine whether stream export of NO3− accurately reflects upland NO3− losses. In this data set, soil solution and stream water [NO3−] were correlated across 40 undisturbed forest watersheds, with stream water [NO3−] typically half (median = 50%) soil solution [NO3−]. A similar relationship was seen in 10 disturbed forest watersheds. However, for 12 watersheds with significant agricultural or urban development, the intercept and slope were both significantly higher than the relationship seen in forest watersheds. Differences in concentration between soil solution or groundwater and stream water may be attributed to biological uptake, microbial processes including denitrification, and/or preferential flow routing. The results of this synthesis are consistent with the hypotheses that undisturbed watersheds have a significant capacity to remove nitrate after it passes below the rooting zone and that land use changes tend to alter the efficiency or the length of watershed flow paths, leading to reductions in nitrate removal and increased stream nitrate concentrations.

Rye–Corn Silage Double‐Cropping Reduces Corn Yield but Improves Environmental Impacts

Recent proliferation of large dairies has prompted concern regarding environmental impacts of associated corn silage production and high‐rate manure application. Our objectives were to compare environmental impacts and forage production of monocrop corn (Zea mays L.) silage and rye (Secale cereal L.)–corn silage double‐crop systems with multiple corn planting dates and high‐rate manure application near Morris, MN. From 2007 to 2009, corn for silage was seeded into a silt loam as a monocrop in early and mid‐May and as a double‐crop after rye in mid‐May and early June. Manure was fall applied annually at average total N and P rates of 393 and 109 kg ha−1, respectively. Double‐cropping reduced total forage dry matter (DM) yield 2 of 3 yr and reduced corn DM yield 15 to 25%. Soil NO3–N to 90 cm accumulated at an average rate of 71 kg N ha−1 yr−1 with monocropping, but accumulation was not observed with double‐cropping. Soil organic C concentration from 0 to 5 cm increased in the monocrop (18%) and double‐crop (26%) systems over 3 yr. Average soil solution NO3–N concentration was high with monocropping (52 mg L−1) and double‐cropping (37 mg L−1), but estimated leaching load averaged only 8 kg ha−1 yr−1. Fall and spring ground cover was often less than 10% with monocropping but was usually greater than 30% with double‐cropping. The primary environmental concerns identified for monocrop corn silage were soil NO3–N buildup and inadequate ground cover. Double‐cropping addressed each concern but often decreased forage production.

Controls on soil solution nitrogen along an altitudinal gradient in the Scottish uplands

Nitrogen (N) deposition continues to threaten upland ecosystems, contributing to acidification, eutrophication and biodiversity loss. We present results from a monitoring study aimed at investigating the fate of this deposited N within a pristine catchment in the Cairngorm Mountains (Scotland). Six sites were established along an elevation gradient (486–908m) spanning the key habitats of temperate maritime uplands. Bulk deposition chemistry, soil carbon content, soil solution chemistry, soil temperature and soil moisture content were monitored over a 5year period. Results were used to assess spatial variability in soil solution N and to investigate the factors and processes driving this variability. Highest soil solution inorganic N concentrations were found in the alpine soils at the top of the hillslope. Soil carbon stock, soil solution dissolved organic carbon (DOC) and factors representing site hydrology were the best predictors of NO3− concentration, with highest concentrations at low productivity sites with low DOC and freely-draining soils. These factors act as proxies for changing net biological uptake and soil/water contact time, and therefore support the hypothesis that spatial variations in soil solution NO3− are controlled by habitat N retention capacity. Soil percent carbon was a better predictor of soil solution inorganic N concentration than mass of soil carbon. NH4+ was less affected by soil hydrology than NO3− and showed the effects of net mineralization inputs, particularly at Racomitrium heath and peaty sites. Soil solution dissolved organic N concentration was strongly related to both DOC and temperature, with a stronger temperature effect at more productive sites. Due to the spatial heterogeneity in N leaching potential, a fine-scale approach to assessing surface water vulnerability to N leaching is recommended over the broad scale, critical loads approach currently in use, particularly for sensitive areas.