Nitrogen pools and fluxes in diversified cropping systems

Achieving high crop yields requires a large supply of plant available nitrogen (N), yet losses of inorganic N from agriculture are deleterious to environmental quality. A significant portion of agricultural N losses could be prevented if large soil inorganic N pools were not needed to satisfy crop N demand. Alternative N management strategies that consider N fluxes like gross N mineralization in addition to N pools should be investigated, as they could conceivably reduce the size of soil inorganic N pools while still providing sufficient N for crop production. Diversified cropping systems may be able to utilize such alternative N management strategies to reduce N losses and increase crop productivity. Characterization of the effects of cropping systems on crop N uptake, soil inorganic N pools, and N fluxes will enable testing of the importance of N dynamics in diverse compared to simple cropping systems. Understanding the relative rates of crop N uptake and inorganic N production by mineralization of soil organic matter could determine the potential for internal N cycling to fulfill crop N demand. Furthermore, if consistent and easy to measure predictors of N mineralization could be identified, estimations of N mineralization could be widely utilized for both research and agricultural management purposes. The Marsden Farm cropping systems experiment compares diverse and simple corn-based cropping systems, and is utilized here to investigate the effects of cropping systems on N pools and fluxes. Over a 12-year period, corn grown in diversified cropping systems required 5.7-fold less synthetic N fertilizer than corn grown in a simple cropping system, yet yielded 4% more grain. It is also likely that nitrate leaching was reduced, as spring soil NO3- concentrations at 1.2 m depth were on average 33% lower in the diversified systems. Further investigations focused on a 2 year period, and revealed that neither soil inorganic N pool size nor potential net N mineralization rate could explain crop N uptake. There was a positive relationship between gross N mineralization and corn N status late in the growing season, but this relationship was consistent across cropping systems and thus did not explain the cropping system effect on yield. Other potential explanations such as corn rooting characteristics, soil moisture status, and corn-microbe interactions should be investigated as causes of the cropping system effect. Gross N mineralization rate was found to be much greater than peak corn N uptake at Marsden, approximately 5-fold higher, which suggests that corn could potentially fulfill much of its N demand by tapping into internal soil N fluxes. However, the crop’s ability to access this N supply will depend on how well it can compete with inorganic N consumption processes such as microbial immobilization, denitrification, and leaching; this topic deserves future research attention. Finally, the Marsden experiment and 5 other cropping systems experiments were used to examine predictors of potential gross and net N mineralization. Results suggested that the quantity and quality of soil organic matter could serve as effective predictors of N mineralization rates. Multiple linear regression models were able to predict both gross N mineralization and net N mineralization (R2=0.8) although the predictors were different for gross and net mineralization, which indicated that different factors influenced these processes. Predictions were valid over the range of sites and management strategies investigated

Can mineralization of soil organic nitrogen meet maize nitrogen demand?

Aims High-yielding maize-based crop systems require maize to take up large quantities of nitrogen over short periods of time. Nitrogen management in conventional crop systems assumes that soil N mineralization alone cannot meet rapid rates of crop N uptake, and thus large pools of inorganic N, typically supplied as fertilizer, are required to meet crop N demand. Net soil N mineralization data support this assumption; net N mineralization rates are typically lower than maize N uptake rates. However, net N mineralization does not fully capture the flux of N from organic to inorganic forms. Gross ammonification may better represent the absolute flux of inorganic N produced by soil N mineralization. Methods Here we utilize a long-term cropping systems experiment in Iowa, USA to compare the peak rate of N accumulation in maize biomass to the rate of inorganic N production through gross ammonification of soil organic N. Results Peak maize N uptake rates averaged 4.4 kg N ha−1 d−1, while gross ammonification rates over the 0–80 cm depth averaged 23 kg N ha−1 d−1. Gross ammonification was highly stratified, with 63% occurring in the 0–20 cm depth and 37% in the 20–80 cm depth. Neither peak maize N uptake rate nor gross ammonification rate differed significantly among three cropping systems with varied rotation lengths and fertilizer inputs. Conclusions Gross ammonification rate was 3.4 to 4.5 times greater than peak maize N uptake across the cropping systems, indicating that inorganic N mineralized from soil organic matter may be able to satisfy a large portion of crop N demand, and that explicit consideration of gross N mineralization may contribute to development of strategies that reduce crop reliance on large soil inorganic N pools that are easily lost to the environment

Nitrogen Fertilizer Suppresses Mineralization of Soil Organic Matter in Maize Agroecosystems

The possibility that N fertilizer increases soil organic matter (SOM) mineralization and, as a result, reduces SOM stocks has led to a great debate about the long-term sustainability of maize-based agroecosystems as well as the best method to estimate fertilizer N use efficiency (FNUE). Much of this debate is because synthetic N fertilizer can positively or negatively affect SOM mineralization via several direct and indirect pathways. Here, we test a series of hypotheses to determine the direction, magnitude, and mechanism of N fertilizer effect on SOM mineralization and discuss the implications for methods to estimate FNUE. We measured the effect of synthetic N fertilizer on SOM mineralization via gross ammonification at two long-term experiments in central and southern Iowa, USA with replicated plots of continuous maize that received one of three “historical” N fertilizer rates (zero, moderate or high) from 1999 to 2014. In 2015, prior to our measurements, we split the historical N fertilizer rate plots into two subplots that received either the site-specific agronomic optimum N rate or zero N fertilizer. At the onset of rapid maize N uptake, N fertilizer reduced gross ammonification by 13–21% (2–5 kg NH4-N ha−1 d−1). A companion laboratory experiment rejected the hypothesis that differences in net primary productivity between fertilized and unfertilized treatments explained the negative effect of N fertilizer on SOM mineralization. Moreover, the NH4+ pool size was negatively correlated with the gross ammonification rate (r2 = 0.85, p \u3c 0.001). Thus, we conclude that NH4+ -N fertilizer had a direct suppressive effect on SOM mineralization. These results demonstrate that the direct effect of N fertilizer on microbial activity can exceed the indirect effects of N fertilizer via large changes in NPP that alter organic matter inputs, soil temperature and moisture content. The magnitude of this effect and specificity to NH4+ -N has significant implications for fertilizer management as well as the measurement and modeling of agroecosystem N dynamics including FNUE

Nitrogen Fertilizer Suppresses Mineralization of Soil Organic Matter in Maize Agroecosystems

The possibility that N fertilizer increases soil organic matter (SOM) mineralization and, as a result, reduces SOM stocks has led to a great debate about the long-term sustainability of maize-based agroecosystems as well as the best method to estimate fertilizer N use efficiency (FNUE). Much of this debate is because synthetic N fertilizer can positively or negatively affect SOM mineralization via several direct and indirect pathways. Here, we test a series of hypotheses to determine the direction, magnitude, and mechanism of N fertilizer effect on SOM mineralization and discuss the implications for methods to estimate FNUE. We measured the effect of synthetic N fertilizer on SOM mineralization via gross ammonification at two long-term experiments in central and southern Iowa, USA with replicated plots of continuous maize that received one of three “historical” N fertilizer rates (zero, moderate or high) from 1999 to 2014. In 2015, prior to our measurements, we split the historical N fertilizer rate plots into two subplots that received either the site-specific agronomic optimum N rate or zero N fertilizer. At the onset of rapid maize N uptake, N fertilizer reduced gross ammonification by 13–21% (2–5 kg NH4-N ha−1 d−1). A companion laboratory experiment rejected the hypothesis that differences in net primary productivity between fertilized and unfertilized treatments explained the negative effect of N fertilizer on SOM mineralization. Moreover, the NH4+ pool size was negatively correlated with the gross ammonification rate (r2 = 0.85, p < 0.001). Thus, we conclude that NH4+-N fertilizer had a direct suppressive effect on SOM mineralization. These results demonstrate that the direct effect of N fertilizer on microbial activity can exceed the indirect effects of N fertilizer via large changes in NPP that alter organic matter inputs, soil temperature and moisture content. The magnitude of this effect and specificity to NH4+-N has significant implications for fertilizer management as well as the measurement and modeling of agroecosystem N dynamics including FNUE

An international laboratory comparison of dissolved organic matter composition by high resolution mass spectrometry: Are we getting the same answer?

High-resolution mass spectrometry (HRMS) has become a vital tool for dissolved organic matter (DOM) characterization. The upward trend in HRMS analysis of DOM presents challenges in data comparison and interpretation among laboratories operating instruments with differing performance and user operating conditions. It is therefore essential that the community establishes metric ranges and compositional trends for data comparison with reference samples so that data can be robustly compared among research groups. To this end, four identically prepared DOM samples were each measured by 16 laboratories, using 17 commercially purchased instruments, using positive-ion and negative-ion mode electrospray ionization (ESI) HRMS analyses. The instruments identified ~1000 common ions in both negative- and positive-ion modes over a wide range of m/z values and chemical space, as determined by van Krevelen diagrams. Calculated metrics of abundance-weighted average indices (H/C, O/C, aromaticity, and m/z) of the commonly detected ions showed that hydrogen saturation and aromaticity were consistent for each reference sample across the instruments, while average mass and oxygenation were more affected by differences in instrument type and settings. In this paper we present 32 metric values for future benchmarking. The metric values were obtained for the four different parameters from four samples in two ionization modes and can be used in future work to evaluate the performance of HRMS instruments

Evolutionary History of the Odd-Nosed Monkeys and the Phylogenetic Position of the Newly Described Myanmar Snub-Nosed Monkey Rhinopithecus strykeri

Odd-nosed monkeys represent one of the two major groups of Asian colobines. Our knowledge about this primate group is still limited as it is highlighted by the recent discovery of a new species in Northern Myanmar. Although a common origin of the group is now widely accepted, the phylogenetic relationships among its genera and species, and the biogeographic processes leading to their current distribution are largely unknown. To address these issues, we have analyzed complete mitochondrial genomes and 12 nuclear loci, including one X chromosomal, six Y chromosomal and five autosomal loci, from all ten odd-nosed monkey species. The gene tree topologies and divergence age estimates derived from different markers were highly similar, but differed in placing various species or haplogroups within the genera Rhinopithecus and Pygathrix. Based on our data, Rhinopithecus represent the most basal lineage, and Nasalis and Simias form closely related sister taxa, suggesting a Northern origin of odd-nosed monkeys and a later invasion into Indochina and Sundaland. According to our divergence age estimates, the lineages leading to the genera Rhinopithecus, Pygathrix and Nasalis+Simias originated in the late Miocene, while differentiation events within these genera and also the split between Nasalis and Simias occurred in the Pleistocene. Observed gene tree discordances between mitochondrial and nuclear datasets, and paraphylies in the mitochondrial dataset for some species of the genera Rhinopithecus and Pygathrix suggest secondary gene flow after the taxa initially diverged. Most likely such events were triggered by dramatic changes in geology and climate within the region. Overall, our study provides the most comprehensive view on odd-nosed monkey evolution and emphasizes that data from differentially inherited markers are crucial to better understand evolutionary relationships and to trace secondary gene flow

Nitrogen pools and fluxes in diversified cropping systems

Achieving high crop yields requires a large supply of plant available nitrogen (N), yet losses of inorganic N from agriculture are deleterious to environmental quality. A significant portion of agricultural N losses could be prevented if large soil inorganic N pools were not needed to satisfy crop N demand. Alternative N management strategies that consider N fluxes like gross N mineralization in addition to N pools should be investigated, as they could conceivably reduce the size of soil inorganic N pools while still providing sufficient N for crop production. Diversified cropping systems may be able to utilize such alternative N management strategies to reduce N losses and increase crop productivity. Characterization of the effects of cropping systems on crop N uptake, soil inorganic N pools, and N fluxes will enable testing of the importance of N dynamics in diverse compared to simple cropping systems. Understanding the relative rates of crop N uptake and inorganic N production by mineralization of soil organic matter could determine the potential for internal N cycling to fulfill crop N demand. Furthermore, if consistent and easy to measure predictors of N mineralization could be identified, estimations of N mineralization could be widely utilized for both research and agricultural management purposes. The Marsden Farm cropping systems experiment compares diverse and simple corn-based cropping systems, and is utilized here to investigate the effects of cropping systems on N pools and fluxes. Over a 12-year period, corn grown in diversified cropping systems required 5.7-fold less synthetic N fertilizer than corn grown in a simple cropping system, yet yielded 4% more grain. It is also likely that nitrate leaching was reduced, as spring soil NO3- concentrations at 1.2 m depth were on average 33% lower in the diversified systems. Further investigations focused on a 2 year period, and revealed that neither soil inorganic N pool size nor potential net N mineralization rate could explain crop N uptake. There was a positive relationship between gross N mineralization and corn N status late in the growing season, but this relationship was consistent across cropping systems and thus did not explain the cropping system effect on yield. Other potential explanations such as corn rooting characteristics, soil moisture status, and corn-microbe interactions should be investigated as causes of the cropping system effect. Gross N mineralization rate was found to be much greater than peak corn N uptake at Marsden, approximately 5-fold higher, which suggests that corn could potentially fulfill much of its N demand by tapping into internal soil N fluxes. However, the crop’s ability to access this N supply will depend on how well it can compete with inorganic N consumption processes such as microbial immobilization, denitrification, and leaching; this topic deserves future research attention. Finally, the Marsden experiment and 5 other cropping systems experiments were used to examine predictors of potential gross and net N mineralization. Results suggested that the quantity and quality of soil organic matter could serve as effective predictors of N mineralization rates. Multiple linear regression models were able to predict both gross N mineralization and net N mineralization (R2=0.8) although the predictors were different for gross and net mineralization, which indicated that different factors influenced these processes. Predictions were valid over the range of sites and management strategies investigated.</p

Can soil nitrogen dynamics explain the yield benefit of crop diversification?

Diversification of grain-based cropping systems with forage legumes is commonly observed to enhance grain yields, yet the specific causes of this benefit remain poorly understood. One proposed cause is greater N availability, particularly late in the growing season, as these systems typically include organic N inputs such as legume residues and manure that may release mineral N over an extended period. In this study, we utilized a long-term cropping systems experiment in Iowa, USA to determine if differences in soil N dynamics could explain greater corn yield and N uptake in diversified cropping systems. The experiment compared a simple 2-year corn-soybean rotation system with two more diverse systems: a 3-year rotation of corn-soybean-oats/red clover and a 4-year rotation of corn-soybean-oats/alfalfa-alfalfa. The simple system relied on inorganic N fertilizers, whereas the diversified systems received a combination of inorganic N fertilizers and composted cattle manure, as well as forage legume residues. Measurements included longterm (12-year) corn yields, corn N uptake, leaf N concentration, and soil inorganic N pools (0-30cm) over two growing seasons, and anaerobic potentially mineralizable N (PMN) and gross ammonification (0-20 cm) over a single growing season. Relative to the simple cropping system, corn yields and maximum corn N content in the diversified cropping systems were enhanced by 4% (0.85 Mg ha-1) and 6-20% (10-28 kg N ha-1), respectively, confirming the benefit of crop diversification. The diversified systems also reduced soil inorganic N pools by an average of 11- 28% (5-13 kg N ha-1) and enhanced anaerobic potentially mineralizable N by 18-33% (13-24 kg N ha-1). However, neither soil inorganic N nor PMN were related to corn N uptake or yield, suggesting that inorganic N pools and net N mineralization were not responsible for differences among treatments in corn yields. After accounting for spatial differences in soil organic C, gross ammonification measured late in the season was positively related to corn leaf N concentration and total N content. However, this relationship was not specific to the diversified cropping systems and thus did not explain the crop diversification effect. Overall, we reject the hypothesis that soil N availability plays a major role in boosting corn yields in the diversified systems, and we suggest several alternative lines of investigation for future research, including crop-microbe interactions and soil physical properties.This is a manuscript of an article published as Osterholz, William R., Matt Liebman, and Michael J. Castellano. "Can soil nitrogen dynamics explain the yield benefit of crop diversification?." Field crops research 219 (2018): 33-42. doi:10.1016/j.fcr.2018.01.026. Posted with permission.This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License

Can mineralization of soil organic nitrogen meet maize nitrogen demand?

Aims High-yielding maize-based crop systems require maize to take up large quantities of nitrogen over short periods of time. Nitrogen management in conventional crop systems assumes that soil N mineralization alone cannot meet rapid rates of crop N uptake, and thus large pools of inorganic N, typically supplied as fertilizer, are required to meet crop N demand. Net soil N mineralization data support this assumption; net N mineralization rates are typically lower than maize N uptake rates. However, net N mineralization does not fully capture the flux of N from organic to inorganic forms. Gross ammonification may better represent the absolute flux of inorganic N produced by soil N mineralization. Methods Here we utilize a long-term cropping systems experiment in Iowa, USA to compare the peak rate of N accumulation in maize biomass to the rate of inorganic N production through gross ammonification of soil organic N. Results Peak maize N uptake rates averaged 4.4 kg N ha−1 d−1, while gross ammonification rates over the 0–80 cm depth averaged 23 kg N ha−1 d−1. Gross ammonification was highly stratified, with 63% occurring in the 0–20 cm depth and 37% in the 20–80 cm depth. Neither peak maize N uptake rate nor gross ammonification rate differed significantly among three cropping systems with varied rotation lengths and fertilizer inputs. Conclusions Gross ammonification rate was 3.4 to 4.5 times greater than peak maize N uptake across the cropping systems, indicating that inorganic N mineralized from soil organic matter may be able to satisfy a large portion of crop N demand, and that explicit consideration of gross N mineralization may contribute to development of strategies that reduce crop reliance on large soil inorganic N pools that are easily lost to the environment.This is a manuscript of an article is published as Osterholz, William R., Oshri Rinot, Matt Liebman, and Michael J. Castellano. "Can mineralization of soil organic nitrogen meet maize nitrogen demand?." Plant and Soil 415, no. 1-2 (2017): 73-84. doi: 10.1007/s11104-016-3137-1. Posted with permission.</p

Establishment and First Year Yield of Interseeded Alfalfa as Influenced by Corn Plant Density and Treatment with Prohexadione, Fungicide and Insecticide

Interseeding alfalfa (Medicago sativa L.) into a silage corn (Zea mays L.) companion crop can increase the yield and profitability of forage production and reduce the risk of nutrient and soil loss from cropland, but unreliable establishment of alfalfa hampers the adoption of this practice on dairy farms. This study evaluated plant survival, foliar health, and dry matter yields of two alfalfa varieties when established in corn sown at populations ranging from about 47,500 to 100,000 plants per ha&minus;1 and when treated with prohexadione (PHD), PHD followed by fungicide and insecticide (PHD-FI), or not treated with agrichemicals. The plant density of alfalfa during establishment was adversely impacted by above average precipitation and high corn populations, but substantially improved by PHD-FI treatment, which limited alfalfa etiolation, disease, and defoliation. First-cut dry-matter yields of interseeded alfalfa after corn were maximized at a stand density of approximately 200 plants m&minus;2 or 850 stems m&minus;2 and total first year yield exceeded conventionally spring-seeded alfalfa by 59 to 75%. Overall, our results indicated that PHD-FI treatment promoted good establishment and subsequent forage production of interseeded alfalfa. Applications of PHD-FI must, however, be fine-tuned, and additional management practices must be developed to ensure both good yields of corn silage and reliable establishment of interseeded alfalfa, especially during wet growing conditions