Grassland-Cropping Rotations: An Avenue for Agricultural Diversification to Reconcile High Production with Environmental Quality.

A need to increase agricultural production across the world to ensure continued food security appears to be at odds with the urgency to reduce the negative environmental impacts of intensive agriculture. Around the world, intensification has been associated with massive simplification and uniformity at all levels of organization, i.e., field, farm, landscape, and region. Therefore, we postulate that negative environmental impacts of modern agriculture are due more to production simplification than to inherent characteristics of agricultural productivity. Thus by enhancing diversity within agricultural systems, it should be possible to reconcile high quantity and quality of food production with environmental quality. Intensification of livestock and cropping systems separately within different specialized regions inevitably leads to unacceptable environmental impacts because of the overly uniform land use system in intensive cereal areas and excessive N-P loads in intensive animal areas. The capacity of grassland ecosystems to couple C and N cycles through microbial-soil-plant interactions as a way for mitigating the environmental impacts of intensive arable cropping system was analyzed in different management options: grazing, cutting, and ley duration, in order to minimize trade-offs between production and the environment. We suggest that integrated crop-livestock systems are an appropriate strategy to enhance diversity. Sod-based rotations can temporally and spatially capture the benefits of leys for minimizing environmental impacts, while still maintaining periods and areas of intensive cropping. Long-term experimental results illustrate the potential of such systems to sequester C in soil and to reduce and control N emissions to the atmosphere and hydrosphere.

From use efficiency to effective use of nitrogen: A dilemma for maize breeding improvement.

Maize (Zea mays L.) breeding is continuously moving forward yield gains for many fields crops, increasing dependency to technology such as high input seed costs and high use of nitrogen (N) fertilizers. For this crop, breeding improvement led to concomitantly enhancing N recovery and uptake but following a similar ratio relative to the plant biomass (W) and nitrogen nutrition index (NNI, as actual to critical N concentration) levels. The aim of this review is to provides new insights related to the true gains of N use efficiency (NUE) for maize over time and to propose new direction to target improvement on the effective use of N. Thus, the increase in fertilizer N for modern hybrids to attain greater yields lead to a greater dependency to N fertilization and potentially increasing the overall environmental risks for N losses associated to this practice. Contrarily to the improvement based on NUE, improving the intrinsic N uptake capacity (more N uptake per unit of biomass) is needed to maximize yield and the effective use of N. These results highlight that crop breeding should refocus to directly target an increase on the effective use of N, increasing the efficiency on using environmental resources while seeking for improving attainable yields. SYNOPSIS: Enhancing resilience of our production systems is critical for food security goals. This review highlights the need of investment on directly targeting improvement of the effective use of N not only to improve efficiency but to reduce the dependency to fertilization and environmental risks.

A global dataset to parametrize critical nitrogen dilution curves for major crop species.

Precise management of crop nitrogen nutrition is essential to maximize yields while limiting pollution risks. For several decades, the critical nitrogen (N) dilution curve - relating plant biomass (W) to N concentration (%N) - has become a key tool for diagnosing plant nutritional status. Increasing number of studies are being conducted to parameterize critical N dilution curves of a wide range of crop species in different environments and N-fertilized conditions. A global synthesis of the resulting data is lacking on this topic. Here, we conduct a systematic review of the experimental data collected worldwide to parametrize critical N dilution curves. The dataset consists of 36 papers containing a total of 4454 observations for 19 major crop species distributed in 16 countries. The key variables of this dataset are the W and %N collected at three or more sampling times, containing three or more fertilizer N rate levels. This dataset can guide the development of generic critical N dilution curves, helps scientists to identify factors influencing plant N status, and leads to the formulation of more robust N recommendations for a broad range of environmental conditions.

Crop Mass and N Status as Prerequisite Covariables for Unraveling Nitrogen Use Efficiency across Genotype-by-Environment-by-Management Scenarios: A Review.

Due to the asymptotic nature of the crop yield response curve to fertilizer N supply, the nitrogen use efficiency (NUE, yield per unit of fertilizer applied) of crops declines as the crop N nutrition becomes less limiting. Therefore, it is difficult to directly compare the NUE of crops according to genotype-by-environment-by-management interactions in the absence of any indication of crop N status. The determination of the nitrogen nutrition index (NNI) allows the estimation of crop N status independently of the N fertilizer application rate. Moreover, the theory of N dilution in crops indicates clearly that crop N uptake is coregulated by (i) soil N availability and (ii) plant growth rate capacity. Thus, according to genotype-by-environment-by-management interactions leading to variation in potential plant growth capacity, N demand for a given soil N supply condition would be different; consequently, the NUE of the crop would be dissimilar. We demonstrate that NUE depends on the crop potential growth rate and N status defined by the crop NNI. Thus, providing proper context to NUE changes needs to be achieved by considering comparisons with similar crop mass and NNI to avoid any misinterpretation. The latter needs to be considered not only when analyzing genotype-by-environment-by-management interactions for NUE but for other resource use efficiency inputs such as water use efficiency (colimitation N-water) under field conditions.

Influence of summer sowing dates, N fertilization and irrigation on autumn VSP accumulation and dynamics of spring regrowth in alfalfa (Medicago sativa L.).

Herbage yield of alfalfa (Medicago sativa L.) depends on forage management or environmental conditions that change C and N resource acquisition, and endogenous plants factors such as root organic reserves and number of active meristems. The aim of this work is to study the influence of two sowing dates in summer (12 July or 9 August), N fertilization (0 or 100 kg ha(-1)) and/or irrigation applied during the first year of alfalfa establishment on (i) the accumulation of N organic reserves (soluble proteins and more specifically vegetative storage protein) in taproots during autumn, (ii) the number of crown axillary meristems present at the end of winter and (iii) the dynamics of spring shoot growth. Delaying the sowing date for one month reduced root growth and root N storage, especially vegetative storage proteins (VSP) during autumn. Irrespective of sowing dates, N fertilization did not affect root biomass, number of crown buds, total root N, root soluble protein or VSP concentrations. By contrast, water deficiency during alfalfa establishment in the early summer reduced both root growth and N reserve accumulation. When spring growth resumed, there is a significant linear relationship between leaf area development and soluble protein and VSP concentrations in taproots, and also the number of crown buds. The results showed that an early sowing date and adequate water status during the summer allowed alfalfa plants to accumulate N reserves by increasing taproot mass and soluble protein concentrations, especially VSPs. This resulted in rapid shoot regrowth rates the following spring.