Simplification of soil biota communities impairs nutrient recycling and enhances above- and belowground nitrogen losses.

Agriculture is a major source of nutrient pollution, posing a threat to the earth system functioning. Factors determining the nutrient use efficiency of plant-soil systems need to be identified to develop strategies to reduce nutrient losses while ensuring crop productivity. The potential of soil biota to tighten nutrient cycles by improving plant nutrition and reducing soil nutrient losses is still poorly understood. We manipulated soil biota communities in outdoor lysimeters, planted maize, continuously collected leachates, and measured N2 O- and N2 -gas emissions after a fertilization pulse to test whether differences in soil biota communities affected nutrient recycling and N losses. Lysimeters with strongly simplified soil biota communities showed reduced crop N (-20%) and P (-58%) uptake, strongly increased N leaching losses (+65%), and gaseous emissions (+97%) of N2 O and N2 . Soil metagenomic analyses revealed differences in the abundance of genes responsible for nutrient uptake, nitrate reduction, and denitrification that helped explain the observed nutrient losses. Soil biota are major drivers of nutrient cycling and reductions in the diversity or abundance of certain groups (e.g. through land-use intensification) can disrupt nutrient cycling, reduce agricultural productivity and nutrient use efficiency, and exacerbate environmental pollution and global warming.

Long-term Changes in Grassland Soil Phosphorus with Fertilizer Application and Withdrawal.

Long-term phosphorus (P) applications can increase soil P concentrations in excess of agronomic optima, posing a risk to water quality. Once fertilization stops, however, it may take time for soil P concentrations to decline. Whereas P fertilization adds orthophosphate, little is known about changes in other soil P forms during P buildup and drawdown. This study examined changes in P pools (total P, Olsen P, Mehlich P, and water-extractable P) and P forms determined by P-nuclear magnetic resonance spectroscopy (P-NMR) in grazed grassland plots from Northern Ireland. Between 1994 and 1999, all plots received 8.3 kg P ha yr with variable rates of nitrogen (100-500 kg N ha yr). From 2000 to 2005, plots received 0, 20, 40, or 80 kg P ha yr and 250 kg N ha yr; from 2005 to 2010, no P fertilizer was applied to any plots. In 2005, soil P pool concentrations at the highest P fertilization rates were significantly elevated compared with those in 2000 but had decreased to 2000 concentrations by 2010. In soils receiving no P, soil P pool concentrations were significantly lower than those in 1994 only in 2010. There were few changes in P forms determined by P-NMR. Orthophosphate followed the same trend observed for the soil P pools; total organic P, total inositol phosphates, and total orthophosphate monoesters and diesters were highest in 2010 in the soil receiving no P fertilizer for 10 yr. For these soils, fertilizer application and cessation influenced inorganic P more than organic P.

Impact of legacy soil phosphorus on losses in drainage and overland flow from grazed grassland soils.

Rates and quantities of legacy soil phosphorus (P) lost from agricultural soils, and the timescales for positive change to water quality, remain unclear. From 2000 to 2004 five 0.2ha grazed grassland plots located on a drumlin hillslope in Northern Ireland, received chemical fertiliser applications of 0, 10, 20, 40, 80kgPha-1yr-1 resulting in soil Olsen P concentrations of 19, 24, 28, 38 and 67mgPL-1, respectively, after which applications ceased. Soil Olsen P and losses to overland flow and drainage were monitored from 2005 to 2011 on an event and weekly flow proportional basis, respectively. Soluble reactive P and total P time series were synchronised with daily rainfall and modelled soil moisture deficits. From 2005 to 2011 soil Olsen P decline was proportional to soil P status with a 43% reduction in the plot at 67mgPL-1 in 2004 and a corresponding 12% reduction in the plot with lowest soil P. However, there was no significant difference in the flow-weighted mean concentration for overland flow among plots, all of which exceeded 0.035mgL-1 in >98% of events. Strong interannual and event variations in losses were observed with up to 65% of P being lost during a single rainfall event. P concentrations in drainage flow were independent of Olsen P and drain efficiency was potentially the primary control on concentrations, with the highest concentrations recorded in the plot at 38mgL-1 Olsen P in 2004 (up to 2.72mgL-1). Hydrological drivers, particularly antecedent soil moisture, had a strong influence on P loss in both overland and drainage flow, with higher concentrations recorded above a soil moisture deficit threshold of 7mm. This study demonstrates that on some soil types, legacy P poses a significant long term threat to water quality, even at agronomically optimum soil P levels.

Confirmation of co-denitrification in grazed grassland.

Pasture-based livestock systems are often associated with losses of reactive forms of nitrogen (N) to the environment. Research has focused on losses to air and water due to the health, economic and environmental impacts of reactive N. Di-nitrogen (N2) emissions are still poorly characterized, both in terms of the processes involved and their magnitude, due to financial and methodological constraints. Relatively few studies have focused on quantifying N2 losses in vivo and fewer still have examined the relative contribution of the different N2 emission processes, particularly in grazed pastures. We used a combination of a high (15)N isotopic enrichment of applied N with a high precision of determination of (15)N isotopic enrichment by isotope-ratio mass spectrometry to measure N2 emissions in the field. We report that 55.8 g N m(-2) (95%, CI 38 to 77 g m(-2)) was emitted as N2 by the process of co-denitrification in pastoral soils over 123 days following urine deposition (100 g N m(-2)), compared to only 1.1 g N m(-2) (0.4 to 2.8 g m(-2)) from denitrification. This study provides strong evidence for co-denitrification as a major N2 production pathway, which has significant implications for understanding the N budgets of pastoral ecosystems.

A review of nitrous oxide mitigation by farm nitrogen management in temperate grassland-based agriculture.

Nitrous oxide (N2O) emission from grassland-based agriculture is an important source of atmospheric N2O. It is hence crucial to explore various solutions including farm nitrogen (N) management to mitigate N2O emissions without sacrificing farm profitability and food supply. This paper reviews major N management practices to lower N2O emission from grassland-based agriculture. Restricted grazing by reducing grazing time is an effective way to decrease N2O emissions from excreta patches. Balancing the protein-to-energy ratios in the diets of ruminants can also decrease N2O emissions from excreta patches. Among the managements of synthetic fertilizer N application, only adjusting fertilizer N rate and slow-released fertilizers are proven to be effective in lowering N2O emissions. Use of bedding materials may increase N2O emissions from animal houses. Manure storage as slurry, manipulating slurry pH to values lower than 6 and storage as solid manure under anaerobic conditions help to reduce N2O emissions during manure storage stage. For manure land application, N2O emissions can be mitigated by reducing manure N inputs to levels that satisfy grass needs. Use of nitrification inhibitors can substantially lower N2O emissions associated with applications of fertilizers and manures and from urine patches. N2O emissions from legume based grasslands are generally lower than fertilizer-based systems. In conclusion, effective measures should be taken at each step during N flow or combined options should be used in order to mitigate N2O emission at the farm level.

Insights into the effect of soil pH on N(2)O and N(2) emissions and denitrifier community size and activity.

The objective of this study was to investigate how changes in soil pH affect the N(2)O and N(2) emissions, denitrification activity, and size of a denitrifier community. We established a field experiment, situated in a grassland area, which consisted of three treatments which were repeatedly amended with a KOH solution (alkaline soil), an H(2)SO(4) solution (acidic soil), or water (natural pH soil) over 10 months. At the site, we determined field N(2)O and N(2) emissions using the (15)N gas flux method and collected soil samples for the measurement of potential denitrification activity and quantification of the size of the denitrifying community by quantitative PCR of the narG, napA, nirS, nirK, and nosZ denitrification genes. Overall, our results indicate that soil pH is of importance in determining the nature of denitrification end products. Thus, we found that the N(2)O/(N(2)O + N(2)) ratio increased with decreasing pH due to changes in the total denitrification activity, while no changes in N(2)O production were observed. Denitrification activity and N(2)O emissions measured under laboratory conditions were correlated with N fluxes in situ and therefore reflected treatment differences in the field. The size of the denitrifying community was uncoupled from in situ N fluxes, but potential denitrification was correlated with the count of NirS denitrifiers. Significant relationships were observed between nirS, napA, and narG gene copy numbers and the N(2)O/(N(2)O + N(2)) ratio, which are difficult to explain. However, this highlights the need for further studies combining analysis of denitrifier ecology and quantification of denitrification end products for a comprehensive understanding of the regulation of N fluxes by denitrification.