Drying Of Topsoil Research Articles

Pushing the limits of C3 intrinsic water use efficiency in Mediterranean semiarid steppes: Responses of a drought‐avoider perennial grass to climate aridification

Abstract Intrinsic water use efficiency (WUEi) reflects the trade‐off between photosynthetic carbon gain and water loss through stomatal conductance and is key for understanding dryland plant responses to climate change. Stipa tenacissima is a perennial tussock C3 grass with an opportunistic, drought‐avoiding water use strategy that dominates arid and semiarid steppes across the western Mediterranean region. However, its ecophysiological responses to aridification and woody shrub encroachment, a major land‐use change in drylands worldwide, are not well‐understood. We investigated the variations in leaf stable isotopes (δ18O, δ13C, δ15N), nutrient concentrations (N, P, K), and culm water content and isotopic composition (δ18O, δ2H) of paired pure‐grass and shrub‐encroached S. tenacissima steppes along a 350 km aridity gradient in Spain (10 sites, 160 individuals). Culm water isotopes revealed that S. tenacissima is a shallow‐rooted grass that depends heavily on recent rainwater for water uptake, which may render it vulnerable to increasingly irregular rainfall combined with faster topsoil drying under climate warming and aridification. With increasing aridity, S. tenacissima enhanced leaf‐level WUEi through more stringent stomatal regulation of plant water flux and carbon assimilation (higher δ13C and δ18O), reaching exceptionally high δ13C values (−23‰ to −21‰) at the most arid steppes. Foliar N concentration was remarkably low across sites regardless of woody shrub encroachment, evidencing severe water and N co‐limitation of photosynthesis and productivity. Shrub encroachment decreased leaf P and K but did not affect S. tenacissima water status. Perennial grass cover decreased markedly with both declining winter rainfall and shrub encroachment suggesting population‐level rather than individual‐level responses of S. tenacissima to these changes. The fundamental physiological constraints of photosynthetic C3 metabolism combined with low foliar N content may hamper the ability of S. tenacissima and other drought‐avoider species with shallow roots to achieve further adaptive improvements in WUEi under increasing climatic stress. A drought‐avoiding water use strategy based on early stomatal closure and photosynthesis suppression during prolonged rainless periods may thus compromise the capacity of semiarid S. tenacissima steppes to maintain perennial grass cover, sustain productivity and cope with ongoing climate aridification at the drier parts of their current distribution. Read the free Plain Language Summary for this article on the Journal blog.

Warming positively promoted community appearance restoration of the degraded alpine meadow although accompanied by topsoil drying.

On-going climate warming is threatening the ecological function of grassland ecosystems. However, whether warming has positive effects on community microhabitats and appearance, especially in degraded grasslands, remains elusive. To address this issue, we conducted a 2-year field experiment on the severely degraded alpine meadow and undegraded alpine meadow with nowarming and warming treatments. Community coverage and height in degraded meadow significantly increased under warming, while these changes were not significant in undegraded meadow. Two-year warming increased the community height of degraded meadow and undegraded meadow by 56.55% and 10.99%, respectively. Warming also increased community coverage by 41.88% in degraded meadow and decreased community coverage by 3.01% in undegraded meadow. Moreover, the response of topsoil temperature to warming was stronger in degraded meadow (6.89%) than in undegraded meadow (-0.26%), while the negative response of topsoil moisture to warming was weaker in degraded meadow (-13.95%) than in undegraded meadow (-20.00%). The SEMs further demonstrated that warming had positive effects on topsoil temperature and community height, while had negative effects on topsoil moisture both in degraded and undegraded meadows. Our results confirm that warming-induced soil drying is an important pathway affecting the community appearance in alpine meadows. These findings highlight that warming has positive effects on community height and coverage and is particularly effective in improving community coverage appearance in severely degraded alpine meadow with topsoil drying.

Root and shoot growth of spring wheat (Triticum aestivum L.) are differently affected by increasing subsoil biopore density when grown under different subsoil moisture

A column experiment with five different pore densities (0, 1, 2, 3, and 4 pores column−1) and two varying moisture regimes (comparatively dry and comparatively moist regime) in the subsoil part of the columns was established. In each pore, Lumbricus terrestris was introduced for 28 days before sowing wheat plants. After 40 days of plant growth, watering was stopped to induce progressive topsoil drying. Parameters describing the shoot hydration, mineral uptake, and aboveground biomass were quantified. Root biomass and root length densities (RLD) were measured separately for six soil layers. Under dry subsoil conditions, plants grown under increasing biopore density showed an increase of the RLD and an improved shoot hydration but the aboveground biomass was unaffected. Since RLD but not root biomass was enhanced, it is assumed that roots were able to explore a larger volume of soil with the same amount of root biomass. Thereby, subsoil water likely was used more efficiently leading to an improved hydration. Under moist subsoil conditions, plants grown with increasing biopore density revealed enhanced shoot biomasses and nutrient uptake while the belowground biomass was unaffected. The improved nutrient uptake can be ascribed to, first, the higher subsoil water availability favoring mass flow driven nutrient uptake, and second, to direct and indirect effects of earthworms on the availability of soil nutrients. It is concluded that high biopore abundancies have the potential to improve not only the belowground but also the aboveground biomass. This, however, largely depends on subsoil moisture.

Quantifying monthly water balance to estimate water deficit in Mayurakshi River basin of Eastern India

Recorded seasonal variation and uneven distribution of rainfall is one of the major issues to the agrarian society and the domestic water users today. For strategic planning of water use, scientific estimation of water balance in different spatio-temporal scale is necessary. For this, the present paper intends to investigate monthly and annual water balance state of Mayurakshi River basin (including 45 sub-basins) using precipitation (P), runoff (Q), evapotranspiration (ET) and recharge/soil moisture. P, Q, ET, respectively, vary from 1625.97 mm to 2037.93 mm, 182.19 mm to 1503.15 mm and 1050 mm to 1500 mm for the entire basin. The monthly pattern shows significant variation attributed to strong seasonality of P and temperature (T). Monsoon months recorded heavy maximum P, Q and actual ET and on the other hand, all these are very poor during pre-monsoon season. Groundwater recharge annually differs spatially, but annual average recharge is 307mmwith main contribution in monsoon months. Water deficit condition prevails in pre-monsoon season; strong groundwater lowering and drying of top soil are some explicit evidences of the same. This seasonal water scarcity withstands against multi-cropping strategies of the agrarian communities. To cope with this emerging situation, pressure on groundwater is mounting but it is the fact that this resource is also highly limited and precious. Quantification of monthly water balance and water deficit will be inputs to the strategy development for sustainable water resource management.

Cover crop effects on maize drought stress and yield

Cover crops have been proposed as a tool for adapting cropping systems to drought stress caused by climate change. However, little research has directly evaluated whether cover crops reduce drought stress in the following cash crop. We grew maize in both ambient and imposed drought conditions following four functionally diverse cover crops and a fallow control in a two-site-year study. We looked for evidence that cover crops reduced drought stress by improving cash crop water access or nitrogen (N) status. The study was embedded in an organic cropping systems trial in Pennsylvania, USA. Cover crops and manure fertilizer were incorporated with full inversion tillage.Overall, cover crops neither ameliorated nor exacerbated drought stress in the following maize crop. There was no interaction between cover crop and moisture treatment in a mixed model ANOVA predicting maize kernel yield in either year. Drought reduced yield by 33 % in 2014 and 17 % in 2015. Cereal rye (Secale cereale L. cv. Aroostook) reduced yield relative to cover crop treatments that contained legumes by up to 43 %. There were no other yield differences among cover crop treatments, including the fallow control. Our results are likely influenced by the short legacy of cover cropping (only one or two preceding cover crops) and the use of full inversion tillage. Cover crops may have greater potential to reduce maize drought stress after long-term use, in systems with less soil disturbance, and when residues are retained on the soil surface. Further research is needed to assess the potential for cover crops and other soil-building practices to reduce drought stress by increasing infiltration rates, improving soil water-holding capacity, enhancing cash crop root exploration, and reducing evaporation from the soil surface.Multiple lines of evidence in this study lead to a new hypothesis: cropping systems that rely on cover crops and other organic amendments for N supply may be at risk for N limitation under drought. First, drought reduced mineralization of N from cover crop residues, especially cereal rye. Second, drought reduced maize N status when topsoil drying was more severe. Since cover crop residues and manure were concentrated in the plow layer (top 20 cm) and maize roots extended into the subsoil, topsoil drying may have reduced N mineralization more than crop water uptake. Further research is needed to determine whether and under what circumstances the hypothesized effect meaningfully reduces crop yield.

Soil disturbance levels, soil water content and the establishment of rainfed chickpea: Mechanised seeding options for smallholder farms in north‐west Bangladesh

AbstractOptimum soil physical conditions for crop establishment are more likely to occur with mechanised row‐sowing using metered seeding and controlled depth placement of seeds than with broadcast sowing, that is common in many traditional South Asian cropping practices. Establishment success is a limitation for post‐rice crops in medium to heavy textured soils in the Eastern Gangetic Plain because sowing coincides with rapid topsoil drying and increases in soil strength. In addition, due to lack of rainfall in the post‐rice season the crop relies on access to stored soil water during vegetative and reproductive growth to achieve adequate yields. Our aim was to determine whether decreased soil disturbance (from full tillage to zero tillage (ZT) to undisturbed soil (Fallow)) and direct seeding (using small‐scale seeders) could 1) enhance chickpea crop establishment by conserving seedbed soil water and 2) alter crop water use of the water stored in the soil profile. The silt loam soil has an estimated volumetric soil water content (θv) of 34% at field capacity. At sowing, θv in the seedbed was between 25% to 29%. This was adequate for successful chickpea crop establishment, but slightly wetter than optimum for tillage. In the wet soil at sowing, there was evidence of smeared furrow walls and poor soil covering of the seed in the seedbed in strip planting (SP; rotating blades in front of tine) and ZT. Uncovered furrows (in ZT; using a tine opener) and the fallow soil lost more surface soil water (9%, 23 days after sowing) than the seedbeds created with greater levels of soil disturbance. In 2008, grain yield of single‐pass shallow tillage (SPST) was greater than SP but in 2009, grain yields of all one‐pass tillage techniques (ZT, SP, SPST) were greater than in the broadcast seeding in fully tilled soils. The improvement in chickpea yields was attributed to greater plant numbers, early plant vigour and root growth. Chickpea roots were found to 60 cm depth in the soil profile, and soil water content was less than the θv of wilting point to this depth at podding. There is evidence that root growth and plant water uptake occur deeper in these soil profiles to achieve the grain yields > 1,000 kg/ha recorded in this study. Mechanised sowing in rows with small‐scale seeders has the capability to overcome establishment and yield limitations for chickpea on medium to heavy soils in the post‐rice season.

Feedbacks between soil penetration resistance, root architecture and water uptake limit water accessibility and crop growth – A vicious circle

Water is the most limiting resource for global crop production. The projected increase of dry spells due to climate change will further increase the problem of water limited crop yields. Besides low water abundance and availability, water limitations also occur due to restricted water accessibility. Soil penetration resistance, which is largely influenced by soil moisture, is the major soil property regulating root elongation and water accessibility. Until now the interactions between soil penetration resistance, root system properties, water uptake and crop productivity are rarely investigated. In the current study we quantified how interactive effects between soil penetration resistance, root architecture and water uptake affect water accessibility and crop productivity in the field. Maize was grown on compacted and uncompacted soil that was either tilled or remained untilled after compaction, which resulted in four treatments with different topsoil penetration resistance. Higher topsoil penetration resistance caused root systems to be shallower. This resulted in increased water uptake from the topsoil and hence topsoil drying, which further increased the penetration resistance in the uppermost soil layer. As a consequence of this feedback, root growth into deeper soil layers, where water would have been available, was reduced and plant growth decreased. Our results demonstrate that soil penetration resistance, root architecture and water uptake are closely interrelated and thereby determine the potential of plants to access soil water pools. Hence, these interactions and their feedbacks on water accessibility and crop productivity have to be accounted for when developing strategies to alleviate water limitations in cropping systems.

Accumulation and partitioning of biomass, nitrogen, phosphorus and potassium among different tissues during the life cycle of rice grown under different water management regimes

Knowledge on periodic uptake, accumulation and allocation of nitrogen (N), phosphorus (P) and potassium (K) in different tissues of a rice plant under different soil moisture management conditions is important when implementing efficient and effective water management practices ensuring sustainable rice production. Rice variety Bg358 was grown in soil columns containing sufficient amounts of N, P and K. Four water management treatments were used; (i) continuous flooding (CF) of soil from crop establishment to maturity, (ii) CF until tillering (4 weeks) and Alternative Wetting and Drying (AWD4) thereafter, (iii) CF until flowering (10 weeks) and AWD thereafter (AWD10), (iv) CF until flowering and Top Soil Drying (TSD) thereafter (TSD10). Harvests were made in 2 weeks intervals. Root growth of AWD4 decreased from 4th week compared with CF, and then enhanced from flowering onwards; thereby developing more roots in the top 20 cm soil layer increasing the root mass ratio. Both N and K uptake continued until maturity while P uptake continued until 2 weeks before maturity, and the amount taken up was highest for K and lowest for P. Moisture limitations substantially reduced tissue P content and retranslocation of P to panicles while the lowest reductions were observed in N. Therefore, internal utilisation was most efficient for P and lowest for N i.e., during grain filling N resorbed only from flag leaves, K from green, dead and flag leaves and only in AWD4, and P from green, flag and dead leaves, and stems. Initially reduced root growth under AWD stimulated after flowering. Soil moisture limitation reduced P uptake greatly and thus had most efficient internal P utilisation mechanisms throughout the life cycle while those of N were the lowest.

Barnyard grass (Echinochloa crus-galli (L.) P.Beauv) is less competitive on rice (Oryza sativa L.) when phosphorus (P) is applied to deeper layers in P-deficient and moisture-limited soils

Barnyard grass (Echinochloa crus-galli (L.) P.Beauv) is a major C4 weed in rice fields. We aimed to explore a management strategy to reduce its competition with rice (Oryza sativa L.) (C3) in low-phosphorus (P) soil with varying soil moisture availability. Therefore, we investigated the effect of P placement at different depths. The main experiment was conducted in pots (15 cm diameter, 70 cm height) filled with P-deficient soil as a replacement series with different plant combinations (rice and barnyard grass monocultures; four plants per pot, and a mixture of rice and barnyard grass; two plants of each species per pot) with two P placement depths (0–5 and 15–20 cm from the soil surface) and three moisture treatments (continuous flooding - CF, alternate wetting and drying - AWD, and top soil drying - TSD from flowering). The pots were maintained until crop maturity. A rhizobox experiment supplemented the main experiment to study the root system plasticity during the initial growth stages (i.e., after 14 and 28 days of growth). Placement of P in a deeper soil layer (15–20 cm) reduced the growth and P uptake of barnyard grass by over 70 and 80 %, respectively, irrespective of the plant combination, and moisture treatments, while such reductions were not observed in rice. Reduced growth and P uptake of barnyard grass were associated with the arrested root elongation even from the very early growth stages failing to search for P applied to 15–20 cm soil layer. In contrast, root elongation of rice increased from early growth stages when P was applied at 15–20 cm soil depth and reached the P-supplied soil layer to ensure P uptake. Average root diameter decreased by 12 %, and the percentage fine root production (i.e., < 0.4 mm diameter) increased by 8 % when P was applied at 15–20 cm deep compared with those at 0–5 cm depth. Phosphorous placement at 15–20 cm soil depth reduced the competitive ability of barnyard grass over rice in P-deficient soils, irrespective of the soil moisture management method.

Topsoil drying combined with increased sulfur supply leads to enhanced aliphatic glucosinolates in Brassica juncea leaves and roots

The decrease of water availability is leading to an urgent demand to reduce the plants’ water supply. This study evaluates the effect of topsoil drying, combined with varying sulfur (S) supply on glucosinolates in Brassica juncea in order to reveal whether a partial root drying may already lead to a drought-induced glucosinolate increase promoted by an enhanced S supply. Without decreasing biomass, topsoil drying initiated an increase in aliphatic glucosinolates in leaves and in topsoil dried roots supported by increased S supply. Simultaneously, abscisic acid was determined, particularly in dehydrated roots, associated with an increased abscisic acid concentration in leaves under topsoil drying. This indicates that the dehydrated roots were the direct interface for the plants’ stress response and that the drought-induced accumulation of aliphatic glucosinolates is related to abscisic acid formation. Indole and aromatic glucosinolates decreased, suggesting that these glucosinolates are less involved in the plants’ response to drought.

Phosphorus accumulation by field-grown canola crops and the potential for deep phosphorus placement in a Mediterranean-type climate

When the bulk of phosphorus (P) is located near the soil surface, spring drying of topsoil in Mediterranean-type climates can reduce P availability to crops and cause potential yield loss. In crop species that require a P supply during spring, deep-placement of P fertiliser has proved an effective method of improving P availability and grain yields; however, the spring P demand of field-grown canola (Brassica napus L.) and therefore potential response to deep P placement is not known. This study investigated the effect of deep- (0.17–0.18 m), conventional- (shallow, 0.07–0.08 m), split- (50% deep, 50% shallow), and nil-P fertiliser treatments on P accumulation and seed yields of canola in two field trials. In addition, a glasshouse experiment with different depths of P fertiliser placement and topsoil drying at different growth stages was conducted. In the glasshouse study, deep P placement resulted in greater P uptake by plants, but did not increase seed yields regardless of the time of topsoil drying. At the relatively high-soil-P field site (canola grown on residual P application from the previous year) in a dry season, there was no biomass response to any residual P fertiliser treatments, and P accumulation had ceased by mid flowering. At the low-P field site, P accumulation continued throughout flowering and silique-filling, and seed yields increased significantly (P ≤ 0.05) in the order of split- &gt; deep- &gt; shallow- &gt; nil-P treatments. Improved seed yields in the split- and deep-P treatments appeared to be the direct result of enhanced P availability; in particular, P uptake during vegetative growth (winter) was higher in the treatments with deep P placement. A greater understanding of P accumulation by field-grown canola in relation to soil P properties is needed for better defining optimum P fertiliser placement recommendations.

The effectiveness of deep placement of fertilisers is determined by crop species and edaphic conditions in Mediterranean-type environments: a review

Much of our knowledge of plant growth in response to soil nutrient supply comes from studies under homogeneous soil conditions. However, the adoption of reduced or nil tillage and shallow banding of fertilisers at the time of seeding causes spatially variable distribution and availability of soil nutrients in agricultural lands. Soil available nutrients, particularly the poorly mobile ones such as phosphorus (P), potassium (K), zinc (Zn), manganese (Mn), and copper (Cu), stratify within the fertilised topsoil. In water-limited environments where the topsoil is prone to drying, soil nutrient stratification may influence nutrient availability and plant uptake because of impeded root growth or reduced diffusion of immobile nutrients to the root surface, or more likely a combination of both factors. Placing fertilisers deeper in the soil profile could increase nutrient acquisition and utilisation by plants as fertiliser nutrients are in the moist soil for a longer part of the growing season. However, the effectiveness of deep placement of fertilisers may also be determined by soil texture, tillage, fertilising history, nutrient mobility, and crop species. In Mediterranean-type climates of southern Australia, a yield response of winter crops to deep fertiliser mostly occurs on infertile sandy soils in low rainfall regions. This contrasts with the responses of winter and summer crops in northern Australia on soils with optimum-to-high nutrients but subjected to rapid and frequent drying of topsoil because of high temperatures and high evaporation demand during the growing season. The pattern of nutrient accumulation by crop species (indeterminate v. determinate) and the mobility of mineral nutrients in the phloem would also modify the effectiveness of deep-placed nutrients under drought. The complexity of plant responses to subsoil nutrition may suggest that before adopting deep fertiliser practice in a paddock it is essential to understand the effects of edaphic and climatic conditions, soil management, and plant–soil interactions in order to achieve maximum yield benefit.

Hydraulic lift by canola plants aids P and K uptake from dry topsoil

The role of hydraulic lift in nutrient uptake from dry topsoil is unclear. Vertically-split root systems were used to measure hydraulic lift and phosphorus (P) and potassium (K) uptake at the rosette and flowering stages of two canola (Brassica napus L.) cultivars (Trigold and Boomer with small and large root systems, respectively) grown in soil low in P and K. Rubidium (Rb) was used as a tracer to examine K uptake from K fertiliser banded in dry topsoil. During vegetative growth, statistically significant hydraulic lift was measured, irrespective of cultivar root size or a specific nutrient (P or K) placed in the topsoil. The Rb tracer revealed significant amounts of K were absorbed from the dry soil, but at rates five-fold lower than from the moist soil. The water lifted could not prevent cessation of P uptake by Boomer and a reduction in P uptake by Trigold. During mid to late flowering, ion uptake from the dry topsoil was impeded despite the continuation of hydraulic lift up to 28 days after the topsoil was dried. Minimal fine root growth following topsoil drying (and reduction in fine root biomass in some treatments) appeared to be the primary cause of impeded nutrient uptake.

Rice straw mulching and nitrogen response of no-till wheat following rice in Bangladesh

Wheat ( Triticum aestivum L.) cultivation in no-till soil of a postrice harvest field utilizes residual soil moisture and reduces the time period from rice harvest to wheat seeding in intensive rice-wheat cropping systems. Some of the major constraints in no-till wheat production are high weed infestation, poor stand establishment due to rapid drying of topsoil and low nitrogen use efficiency (NUE). A field experiment was conducted at the research farm of the Wheat Research Centre, Dinajpur, Bangladesh, for two consecutive years to overcome those constraints, to evaluate rice straw as mulch, and to determine the optimum application rate of nitrogen (N) for no-till wheat. The treatments included 12 factorial combinations of three levels of mulching: no mulch (M 0), surface application of rice straw mulch at 4.0 Mg ha −1 that was withdrawn at 20 days after sowing (M 1), the same level of mulch as M 1 but allowed to be retained on the soil surface (M 2), and four nitrogen levels (control 80, 120 and 160 kg ha −1). Rice straw mulching had a significant effect on conserving initial soil moisture and reducing weed growth. Root length density and root weight density of wheat were positively influenced both by straw mulching and N levels. N uptake and apparent nitrogen recovery of applied N fertilizer were higher in mulch treatments M 1 and M 2 as compared to M 0. Also mulch treatment of M 1 and M 2 were equally effective at conserving soil moisture, suppressing growth of weed flora, promoting root development and thereby improved grain yield of no-till wheat. N application of 120 kg ha −1 with straw mulch was found to be suitable for no-till wheat in experimental field condition.

The influence of water-content gradient and structure anisotropy on soil compressibility

Most attempts to model compaction have used simplistic homogeneous and isotropic profiles of soil structure, and sometimes water content. However, anisotropy of structure and water content are always found in real agricultural soils. In this paper, the distributions with depth of initial water content and bulk density are taken into account to analyse the compaction process of soil assuming that the soil profile may be partitioned in a multilayer system, where each layer is defined as homogeneous and isotropic in regard to its mechanical behaviour. In the first field experiment, bulk density measurements, after wheeling at different tyre inflation pressure, showed that a slight topsoil drying, prior to wheeling, enhances the effectiveness of low pressure tyres in preventing excess compaction. In the second field experiment, compression curves, obtained in situ with a sinkage plate apparatus combined with a gamma-ray probe, showed that it was possible to analyse the compaction process of a multilayer system, defined by its bulk density, considering the compression process of the whole system as the result of an interaction between the compressibility of each layer.

Spontaneous combustion of coal

Cover crops have been proposed as a tool for adapting cropping systems to drought stress caused by climate change. However, little research has directly evaluated whether cover crops reduce drought stress in the following cash crop. We grew maize in both ambient and imposed drought conditions following four functionally diverse cover crops and a fallow control in a two-site-year study. We looked for evidence that cover crops reduced drought stress by improving cash crop water access or nitrogen (N) status. The study was embedded in an organic cropping systems trial in Pennsylvania, USA. Cover crops and manure fertilizer were incorporated with full inversion tillage.Overall, cover crops neither ameliorated nor exacerbated drought stress in the following maize crop. There was no interaction between cover crop and moisture treatment in a mixed model ANOVA predicting maize kernel yield in either year. Drought reduced yield by 33 % in 2014 and 17 % in 2015. Cereal rye (Secale cereale L. cv. Aroostook) reduced yield relative to cover crop treatments that contained legumes by up to 43 %. There were no other yield differences among cover crop treatments, including the fallow control. Our results are likely influenced by the short legacy of cover cropping (only one or two preceding cover crops) and the use of full inversion tillage. Cover crops may have greater potential to reduce maize drought stress after long-term use, in systems with less soil disturbance, and when residues are retained on the soil surface. Further research is needed to assess the potential for cover crops and other soil-building practices to reduce drought stress by increasing infiltration rates, improving soil water-holding capacity, enhancing cash crop root exploration, and reducing evaporation from the soil surface.Multiple lines of evidence in this study lead to a new hypothesis: cropping systems that rely on cover crops and other organic amendments for N supply may be at risk for N limitation under drought. First, drought reduced mineralization of N from cover crop residues, especially cereal rye. Second, drought reduced maize N status when topsoil drying was more severe. Since cover crop residues and manure were concentrated in the plow layer (top 20 cm) and maize roots extended into the subsoil, topsoil drying may have reduced N mineralization more than crop water uptake. Further research is needed to determine whether and under what circumstances the hypothesized effect meaningfully reduces crop yield.