Nitrogen yield advantage from grass-legume mixtures is robust over a wide range of legume proportions and environmental conditions.

Current challenges to global food security require sustainable intensification of agriculture through initiatives that include more efficient use of nitrogen (N), increased protein self-sufficiency through homegrown crops, and reduced N losses to the environment. Such challenges were addressed in a continental-scale field experiment conducted over 3 years, in which the amount of total nitrogen yield (Ntot ) and the gain of N yield in mixtures as compared to grass monocultures (Ngainmix ) was quantified from four-species grass-legume stands with greatly varying legume proportions. Stands consisted of monocultures and mixtures of two N2 -fixing legumes and two nonfixing grasses. The amount of Ntot of mixtures was significantly greater (P ≤ 0.05) than that of grass monocultures at the majority of evaluated sites in all 3 years. Ntot and thus Ngainmix increased with increasing legume proportion up to one-third of legumes. With higher legume percentages, Ntot and Ngainmix did not continue to increase. Thus, across sites and years, mixtures with one-third proportion of legumes attained ~95% of the maximum Ntot acquired by any stand and had 57% higher Ntot than grass monocultures. Realized legume proportion in stands and the relative N gain in mixture (Ngainmix /Ntot in mixture) were most severely impaired by minimum site temperature (R = 0.70, P = 0.003 for legume proportion; R = 0.64, P = 0.010 for Ngainmix /Ntot in mixture). Nevertheless, the relative N gain in mixture was not correlated to site productivity (P = 0.500), suggesting that, within climatic restrictions, balanced grass-legume mixtures can benefit from comparable relative gains in N yield across largely differing productivity levels. We conclude that the use of grass-legume mixtures can substantially contribute to resource-efficient agricultural grassland systems over a wide range of productivity levels, implying important savings in N fertilizers and thus greenhouse gas emissions and a considerable potential for climate change mitigation.

Changes in soil microbial biomass and residual indices as ecological indicators of land use change in temperate permanent grassland.

The relationship between microbial biomass, residues and their contribution to microbial turnover is important to understand ecosystem C storage. The effects of permanent grassland (100 % ryegrass--PG), conversion to modified grassland (mixture of grass and clover--MG) or maize monoculture (MM) on the dynamics of soil organic C (SOC), microbial biomass, fungal ergosterol and microbial residues (bacterial muramic acid and fungal glucosamine) were investigated. Cattle slurry was applied to quantify the effects of fertilisation on microbial residues and functional diversity of microbial community across land use types. Slurry application significantly increased the stocks of microbial biomass C and S and especially led to a shift in microbial residues towards bacterial tissue. The MM treatment decreased the stocks of SOC, microbial biomass C, N and S and microbial residues compared with the PG and MG treatments at 0-40 cm depth. The MM treatment led to a greater accumulation of saprotrophic fungi, as indicated by the higher ergosterol-to-microbial biomass C ratio and lower microbial biomass C/S ratio compared with the grassland treatments. The absence of a white clover population in the PG treatment caused a greater accumulation of fungal residues (presumably arbuscular mycorrhizal fungi (AMF), which do not contain ergosterol but glucosamine), as indicated by the significantly higher fungal C-to-bacterial C ratio and lower ergosterol-to-microbial biomass C ratio compared with the MG treatment. In addition to these microbial biomass and residual indices, the community level physiological profiles (CLPP) demonstrated distinct differences between the PG and MG treatments, suggesting the potential of these measurements to act as an integrative indicator of soil functioning.

Environmental Impact of Rotationally Grazed Pastures at Different Management Intensities in South Africa.

Nitrogen fertilization, irrigation and concentrate feeding are important factors in rotational pasture management for dairy farms in South Africa. The extent to which these factors affect environmental efficiency is subject to current and intense debate among scientists. A three-year field study was conducted to investigate the yield response of different N-fertilizer treatments (0 (N0), 220 (N20), 440 (N40), 660 (N60) and 880 (N80) kg N ha-1 year-1) on grazed pastures and to calculate the carbon footprint (CF) of milk produced. Excessive N-fertilization (N60 and N80) did not increase herbage dry matter and energy yields from pastures. However, N80 indicated the highest N-yield but at the same time also the highest N surpluses at field level. A maximum fertilizer rate of 220 kg ha-1 year-1 (in addition to excreted N from grazing animals) appears sufficient to ensure adequate herbage yields (~20 t DM ha-1 year-1) with a slightly positive field-N-balance. This amount will prevent the depletion of soil C and N, with low N losses to the environment, where adequate milk yields of ~17 t ECM ha-1 with a low CF (~1.3 kg CO2 kg ECM-1) are reached. Methane from enteric fermentation (~49% ± 3.3) and N2O (~16% ± 3.2) emissions from irrigated pastures were the main contributors to the CF. A further CF reduction can be achieved by improved N-fertilization planning, low emission irrigation techniques and strategies to limit N2O emissions from pasture soils in South Africa.

Assessing the Potential of Diverse Forage Mixtures to Reduce Enteric Methane Emissions In Vitro.

Methane emissions from ruminants are a major contributor to agricultural greenhouse gas emissions. Thus, eight different forage species were combined in binary mixtures with Lolium perenne in increasing proportions, in vitro, to determine their methane reduction potential in ruminants. Species were sampled in two consecutive years where possible. The aims were: a) to determine if mixtures with specific forages, particularly those rich in plant specialized metabolites (PSM), can reduce methane emissions compared to ryegrass monocultures, b) to identify whether there is a linear-dose effect relationship in methane emissions from the legume or herb addition, and c) whether these effects are maintained across sampling years. Results showed that all dicot species studied, including the non-tannin-containing species, reduced methane production. The tannin-rich species, Sanguisorba minor and Lotus pedunculatus, showed the greatest methane reduction potential of up to 33%. Due to concomitant reductions in the forage digestibility, Cichorium intybus yielded the lowest methane emissions per digestible forage unit. Contrary to total gas production, methane production was less predictable, with a tendency for the lowest methane production being obtained with a 67.5% share of the legume or herb partner species. Thus, linear increments in the partner species share did not result in linear changes in methane concentration. The methane reduction potential differed across sampling years, but the species ranking in methane concentration was stable.

Yield Progress in Forage Maize in NW Europe-Breeding Progress or Climate Change Effects?

Yield increases in forage maize (Zea mays L.) in NW Europe over time are well documented. The driving causes for these, however, remain unclear as there is little information available regarding the role of plant traits triggering this yield progress. Ten different hybrids from the same maturity group, which have typically been cultivated in Northwest Germany from 1970 to recent and are thus representing breeding progress over four decades, were selected for a 2-year field study in northern Germany. Traits that were investigated included leaf area index, leaf architecture, photosynthesis, radiation use efficiency, root mass, root length density, and turnover. Based on a mixed model analysis with these traits as co-variates, parameters related to leaf characteristics, in particular the number and length of leaves, the radiation use efficiency, and the leaf orientation, were identified as most influential on the yield progress (0.13 tons ha-1 year-1). In contrast to our hypothesis, root biomass only increased negligibly in newer hybrids compared to older ones, confirming the 'functional equilibrium' theory for high input production systems. Due to an abundance of nutrients and water in such high input systems, there is no incentive for breeders to select for carbon partitioning toward the rooting system. Breeding evidence to increase forage quality were also negligible, with no change in cob starch concentration, forage digestibility, nor NDF content and NDF digestibility. The observed increase in yield over the last four decades is due to a combination of increased temperature sums (~240 GDD within 40 years), and a higher radiation interception and radiation use efficiency. This higher radiation interception was driven by an increased leaf area index, with a higher number of leaves (16 instead of 14 leaves within 40 years) and longer leaves of newer compared to older hybrids. Future selection and adaptation of maize hybrids to changing environmental conditions are likely to be the key for high productivity and quality and for the economic viability of maize growing and expansion in Northern Europe.