Soil-derived Nitrogen Research Articles

Elevated atmospheric CO2 and warming enhance the acquisition of soil-derived nitrogen rather than urea fertilizer by rice cultivars

Plant nitrogen (N) acquisition is essential to both crop growth and yield. A rational N management strategy for agricultural systems under climate change is required but a knowledge gap still exists regarding plant N uptake and N origin (soil-derived N or fertilizer-derived N) in response to elevated atmospheric CO2 and warming. Our study investigated the responses of soil- or fertilizer-derived N uptake and yield of different cultivars of rice (Oryza sativa L.) to climate change. Five cultivars in the pedigree of a widely-grown rice cultivar Wuyoudao1 were supplied with and without urea, a commonly used N fertilizer in paddy soils and grown in open-top chambers under elevated CO2 (700 ppm) and warming (2°C higher than the air temperature). Plant N origins were traced using 15N-labeling technique (urea of 5% 15N atom). Compared to the control, elevated CO2 and warming increased N uptake by 17%, irrespective of N supply. Soil-N rather than fertilizer-N was the source of the increased N uptake. The increased soil-N uptake resulted in the enhancement of rice yield under climate change. Urea application did not alter the yield response to elevated CO2 and warming compared to the non-N supply, but did stimulate plant uptake of the soil-derived N. Although genetic improvement of rice germplasm resulted in an increase in plant N acquisition and yield, the history of crop breeding since 1935 did not alter the climate-change-induced response in terms of plant N acquisition. Our results suggest that climate change may lead to the depletion of the recalcitrant soil N pool in paddy soils, and that fertilizer-N-use efficiency may need to be factored into future breeding for rice genotypes adapting well to climate change.

Grain legume-cereal intercropping enhances the use of soil-derived and biologically fixed nitrogen in temperate agroecosystems. A meta-analysis

Abstract Grain legumes are known for their benefits to deliver ecosystem services on provisioning of protein-rich food and feed, reducing greenhouse gas emissions through the symbiotic nitrogen fixation function and diversification of cropping systems. Intercropping is an agroecological practice in which two or more crop species are grown simultaneously in the same field, thereby maximizing the use of resources to enhance yields in low input systems and the resilience of cropping systems. We quantified the effect of grain legume-cereal intercropping on the use of N resources in temperate agroecosystems, focusing on dinitrogen (N2) fixation and soil-derived nitrogen acquisition using a meta-analysis of 29 field-scale studies. We estimated and compared effects of different intercrop compositions (proportion of each species in the intercrops), fertilization rates, crop species, soil properties, and other management practices on the symbiotic N2 fixation and the acquisition of soil-derived nitrogen by the cereals and grain legumes. The proportion of N derived from N2 fixation was on average 14 % (95 % CI = [11, 16]) higher in intercropped grain legumes (76 %) compared to legume sole crops (66 %). On the other hand, intercropping reduced the amount of N2 fixed (kg ha−1) by about 15 %, when N2 fixation in inter- and sole cropped legumes was expressed at equivalent density by compensating for the sown legume proportion in intercrops relative to their sole crop sowing rate. The results were mainly influenced by the intercrop composition, legumes species and the method used to quantify N2 fixation. Soil-derived nitrogen acquisition in intercropped grain legumes was significantly reduced (−47 %, 95 % CI = [−56, −36]) compared to sole crop legumes, expressed at equivalent density, while the soil N acquired by intercropped cereals was much higher (+61 %, 95 % CI = [24, 108]) than in sole crop cereals. Total soil N acquisition (legume + cereal) was significantly higher in intercrops than in legume sole crops (+25 %, 95 % CI = [1, 54]), while there was no significant difference between intercrops and cereal sole crops. The meta-analysis confirms and highlights that intercropping consistently stimulates complementary N use between legumes and cereals by increasing N2 fixation by grain legumes and increasing soil N acquisition in cereals. Based on the results of this analysis it would be suggested that cropping systems diversification via intercropping can be used for simultaneous production of both cereals and grain legumes, while increasing the use of N-sources and reducing external inputs of N fertilizers, thereby enhancing the sustainability of agriculture.

The role of rhizosphere pH in regulating the rhizosphere priming effect and implications for the availability of soil-derived nitrogen to plants.

A comprehensive understanding of the rhizosphere priming effect (RPE) on the decomposition of soil organic carbon (SOC) requires an integration of many factors. It is unclear how N form-induced change in soil pH affects the RPE and SOC sequestration. This study compared the change in the RPE under supply of NO3-N and NH4-N. The effect of the RPE on the mineralization of soil N and hence its availability to plant and microbes was also examined using a 15N-labelled N source. The supply of NH4-N decreased rhizosphere pH by 0.16-0.38 units, and resulted in a decreased or negative RPE. In contrast, NO3-N nutrition increased rhizosphere pH by 0.19-0.78 units, and led to a persistently positive RPE. The amounts of rhizosphere-primed C were positively correlated with rhizosphere pH. Rhizosphere pH affected the RPE mainly through influencing microbial biomass, activity and utilization of root exudates, and the availability of SOC to microbes. Furthermore, the amount of rhizosphere primed C correlated negatively with microbial biomass atom% 15N (R2 0.77-0.98, n = 12), suggesting that microbes in the rhizosphere acted as the immediate sink for N released from enhanced SOC decomposition via the RPE. N form was an important factor affecting the magnitude and direction of the RPE via its effect on rhizosphere pH. Rhizosphere pH needs to be considered in SOC and RPE modelling.

Limitations of the 15N natural abundance method for estimating biological nitrogen fixation in Amazonian forest legumes

Summary This study evaluates the 15N natural abundance method for estimating biological nitrogen fixation (BNF) in 12 secondary and five primary forest sites in central Amazonia. Investigations were based on spatially systematic sampling of 20 potentially N2-fixing legume tree and liana species, and of a range of differing references representing the ‘soil-derived nitrogen’. The absence of systematic differences in the δ15N-signals of potential N2-fixers and reference plants resulted in many invalid % Ndfa-estimates (i.e., the proportion of foliar N derived from BNF). Whereas the % Ndfa-estimates based on non-legumes were randomly distributed around zero, the use of non-N2-fixing legumes resulted in largely positive % Ndfa-estimates. Grouping of reference species into species types was a valid strategy to reduce the risk of atypical values. Leaf litter was an inadequate measure of soil-derived nitrogen. The impact of the B-value was low or moderate at best, the percentage of negative % Ndfa-estimates remained unaltered. The manner of individually pairing putative N2-fixers with reference plants only marginally affected the % Ndfa-estimates, the assumption of an overall site mean reference δ15N-signal was sufficient. Since legumes were aggregated in clusters, BNF locally affected the surrounding soil nitrogen in secondary regrowth. Negative correlations between the vegetation shares of potential N2-fixers and leaf litter δ15N-signals indicate 15N-dilution from the input side. Interpolated reference plant δ15N-signals appear low in the surroundings of legume clusters in some of the older regrowth sites, confirming such 15N-dilution. We conclude that, with our current state of knowledge, the 15N natural abundance method fails to quantitatively estimate BNF in heterogeneous forests. Future research should focus on the search for a more direct measure of the plant-available δ15N-signal of the soil, as well as on plant–soil interactions and the resulting small-scale isotopic heterogeneity.

Uptake, partitioning and utilization of carbon and nitrogen in the phloem bleeding tree, Tasmanian blue gum (Eucalyptus globulus)

This paper originates from a presentation at the International Conference on Assimilate Transport and Partitioning, Newcastle, NSW, August 1999 An empirical modelling procedure was employed to follow uptake, transport and utilization of photo-assimilated carbon (C) and soil-derived nitrogen (N) over a 19-d period (November 1998) in 2-year-old plantation-grown trees of Eucalyptus globulus Labill. Models utilized data for gains and losses of C and N in dry matter (DM) of tree parts, CO2 exchanges and transpiration of foliage, respiratory losses of stems and roots, C:N weight ratios of xylem and phloem sap collected at different sites within the system, and phloem sap sugar concentration gradients along trunks and branches to indicate directions of assimilate flow. The model for C depicted the fate of exported fixed C from four levels of branches on the shoot system, cycling of 16% of the C supplied from shoot to root back to the shoot in xylem, major involvement of xylem-derived C in nourishment of rapidly growing branches, and a net daily respiratory output per tree equivalent to 39% of its net daytime photosynthetic gain in C by foliage. The model for N showed that upper growing shoot parts gained more N mobilized from lower branches than was being acquired from soil. It also indicated high rates of cycling of N through mature foliage, effective retention of xylem-derived N by growing branches and apices, and feedback of substantial amounts of phloem-exported N from lower branches into xylem moving further up the trunk. Transpiration loss per tree was equivalent to 272 mL g–1 DM accumulated. Data are discussed in relation to similarly executed C:N partitioning studies on herbaceous annual species.

Nitrogen and water flows under pasture - wheat and lupin - wheat rotations in deep sands in Western Australia. 1. Nitrogen fixation in legumes, net N mineralisation,and utilisation of soil-derived nitrogen

Detailed studies on the eciency with which pastures and crops use soil-derived nitrogen (N) in southern Australia are limited. Inefficiencies in the N cycle are indicated by wide spread soilacidification and low N status in wheat grain. The aims of this study were to document rates of N2 fixation by subterranean clover-based pastures and narrow-leaf lupin, plant uptake of soil-derived N, mineralisation of organic N during legume and cereal phases, and export of N from pastures, lupin,and wheat in relation to climate and soil water. These measurements were undertaken in a rotation experiment conducted on a deep sand located in the northern wheat belt of Western Australia at a site with a long-term average rainfall of 460 mm. The rotations examined over 3 years were 2 years pasture-wheat and lupin-wheat. The 15N natural abundance technique was used to differentiate soil-derived N from atmospheric Nin legumes. Biomass production, grain yields, and N contents were standard plant measurements in all treatments. Net N mineralisation between growing seasons was as certained by measuring changes in soil inorganic N to 1·5 m. Growing season net N mineralisation was determined using an in situ method in which soil cores were isolated from plant roots. Anion exchange resin was used to trap NO-3 leached below the depth of the soil cores. Nitrogen fixation by subterranean clover in a mixed pasture ranged from 29 to 162 kg N/ha whereas N2 fixation by lupins was less variable, ranging from 90 to 151 kg N/ha. Pastures were large consumers of soil-derived N (range 58-154 kg N/ha), with capeweed being the most important sink (range 38-120 kg N/ha). In comparison, wheat and lupins were inefficient users of soil N, removing 29-51 kg N/ha within a season. Another 31-67 kg N/ha of inorganic N in soil was not utilised by wheat or lupin. Annual net N mineralisation ranged from 80 to 130 kg N, confirming the high rate of decomposition of organic matter in the sandy soil. Mineralisation over summer and autumn, when crop and pastures were not grown, supplied ~25% of the inorganic N produced in soil profiles in 1995 and 20-40% in1996. The study indicated that legumes used in rotations with cereals on deep sands were able to add adequate organic N to soil to insure rates of net N mineralisation sufficient to support cereal yieldsin excess of current shire averages. However, in practice, the asynchrony in supply and demand for N resulted in the inefficient use of soil-derived N by wheat.

Estimation of potentially mineralisable nitrogen in soil by KCl extraction

The mineral nitrogen extractable from 12 Scottish arable soils by boiling for 1 hour with 1 M KCl solution was compared with the quantities taken up by ryegrass, barley and oats in pot tests. High correlations were obtained for ryegrass and oats (r2=0.85 and 0.79, respectively), with a slightly lower correlation (r2=0.64) for barley. When the results for one anomalous soil were eliminated from the analysis, all r2 values were 0.81 or higher. The results confirmed earlier work indicating that this method may prove useful for predicting crop uptake of soil-derived nitrogen.

Symbiotically-fixed and soil-derived nitrogen in legumes grown in pots in soils with different amounts of available nitrate

Symbiotically-fixed and soil-derived nitrogen have been measured in pot experiments for Medicago littoralis (medic), grown alone or with Lolium multiflorum (ryegrass) and for Pisum sativum (field pea). The four soils used contained organic matter labelled with 15N, and differed in their capacities to release available N. During a 4–12-week incubation each released inorganic N (NO − 3) of approximately constant 15N atom% enrichment. In one soil, the mineralized N was supplemented by 15NO − 3 of similar 15N atom% enrichment. Incubation of soils under intermittently moist and dry conditions increased N mineralization rates, but did not affect the 15N atom% enrichments of the released N. For all soils and treatments the amounts of soil-derived N taken up by plants equalled the amounts of available N in moist incubated, unplanted soils. The enrichment of ryegrass root N grown alone or with medic was slightly but consistently less than that of top N. Nitrogen of the legume nodules and pods (peas) was least enriched, followed by N of legume stems, leaves and roots; the 15N atom% enrichments of root N were 4–5 times those of nodule N. Peas generally outyielded and fixed more N than medic grown alone. Medic grown with ryegrass yielded least and fixed least N. For unamended soils, yields of legume dry matter and amounts of N fixed were greatest in Roseworthy or Avon sandy loam soils and least in Northfield clay loam. Addition of 15NO − 3 to Avon soil decreased N fixed by peas and by medic grown alone or with ryegrass. For this soil, soil-derived N of plant tops exceeded fixed N of roots, even for unamended soil where fixation by legumes was relatively high. Thus, complete removal of plant tops would have produced a net loss of N from the soil, the net loss increasing with increasing amounts of 15NO − 3 added.