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Plant Ecology for Nitrous Oxide Mitigation and Sustainable Productivity

Periodic Reporting for period 1 - ECONOMY (Plant Ecology for Nitrous Oxide Mitigation and Sustainable Productivity)

Reporting period: 2016-02-01 to 2018-01-31

For millennia the productivity of most agricultural systems has been limited by the supply of nitrogen (N). Due to this limited availability, natural processes were highly efficient in using mineral N as a resource. However, since application of artificial N fertilizers became widespread in the 1950’s, humans have disrupted the global N cycle at an immense pace, increasing twofold the N inputs entering the Earth’s soil. When N fertilizers are applied to agricultural soils, around half of the N is taken up by the plants leading to higher crop productivity. The other half is lost to surface waters or the atmosphere with detrimental effects on the environment. An important N-loss is the potent ozone-depleting greenhouse gas nitrous oxide (N2O), which is mainly produced through microbial nitrification and denitrification. Grasslands represent 68% of agricultural land across the globe and thus play a pivotal role in the N cycle. It is therefore paramount to identify ways to mitigate N2O emissions from intensive grasslands without compromising high quality food supply.

Research conducted in natural ecosystems indicates that increasing plant species richness in a plant community may augment complementary in time and space for soil nutrient acquisition, enhancing biomass productivity. In intensively managed grasslands this may reduce N2O emissions due to increased N uptake by vegetation and hence reduced availability of soil mineral N for nitrifiers and denitrifiers. However, other studies have shown that targeted selection of species with specific traits or trait diversity may be more important than increasing species richness per se in terms of enhancing complementarity and N use efficiency. In combination with the current understanding of the microbiology behind soil N2O emissions and of plant-trait based ecology, these findings provide a promising framework to develop a novel N2O mitigation strategy.

Climate change has a major impact on N2O emissions because N losses and primary productivity strongly respond to climatic conditions. However, this effect may be regulated to some extent by the presence of specific plants or plant combinations, as there is now increasing evidence that some of the most significant effects of climate change on ecosystem N dynamics are mediated via plants and their interactions with soil microorganisms. Within the context of climate change, the intensification of weather extremes has emerged as one of the most important aspects in terms of consequences for ecological systems and for human welfare. The debate over the last few years has shifted from an analysis of trends to a realization that extreme events rather than average trends may exert the most important controls on soil-plant and plant-plant interactions and ecosystem functioning. Yet, the outcome of such weather-driven interactions in terms of N2O emissions and the mechanisms involved remain poorly understood.

The overall objective of this project was to reveal how plants and plant interactions via their traits and trait combinations can be used to reduce N2O emissions under current and future climatic conditions.
Previous research had shown that compared to species with conservative strategies, species with acquisitive strategies have higher N uptake when there is ample N in the soil, but also trigger N mineralization when soil N is limiting. Therefore, we hypothesized that compared to conservative species, species with acquisitive traits would reduce N2O emissions after a high N addition; and that species with conservative traits would have lower N2O emissions than acquisitive plants if there is no high N addition. This was tested in a greenhouse experiment using monocultures of six grass species with differing above- and below-ground traits, growing across a gradient of soil N availability. We found that acquisitive species reduced N2O emissions at all levels of N availability, produced higher biomass and showed larger N uptake. As such, acquisitive species had almost 90% lower N2O emissions per unit of N uptake than conservative species. Further analyses revealed that specific leaf area and root length density were key traits regulating the effects of plants on N2O emission and biomass productivity.

In a subsequent 2-year field experiment we translated our findings into realistic conditions, covering interactions between plants and including legume species due to their importance to improve fodder quality and their particular role in N cycling. The results of this experiment are been analysed and will be made public soon. One of the most pertinent questions this experiment will answer is how the role of plants in N-cycling is mediated by changes in soil microbial communities, and whether such changes can be linked to plant functional traits. We will also show if plant community effects on several soil biotic factors represent indirect and little studied mechanisms through which plants may modify soil N cycling in intensive grasslands.

In a greenhouse study using intact monoliths from the field experiment, we are currently evaluating the effect of a climate change-induced disturbance (flooding) on the N2O emissions and productivity of intensively managed grasslands across a plant diversity gradient. We are testing the hypothesis that the negative effects of flooding could be mitigated with increasing plant diversity due to a greater functional diversity in traits related to nutrient acquisition (lowering N2O emissions), and a greater resilience of the plant community to flooding (maximizing productivity under unfavourable conditions).

A review study is been conducted, in which we argue that combining plants based on their functional traits may provide an overlooked opportunity to improve the amount of N retained by plants in intensive agroecosystems. Then we illustrate associated benefits of this approach for yield stability, resilience, and agroecosystem multifunctionality. Finally, we will propose optimum plant combinations for enhanced N-cycling following a trait-based approach.
The results of this project provide the first framework to understand the mechanisms through which plants modulate N2O emissions, pointing the way to develop productive grasslands that contribute optimally to climate change mitigation. Our research is highly relevant for agronomists, as we will recommend plant combinations with high productivity and high resilience to climatic stressors. Through the choice of plant species the research outcomes will be relatively easy to translate into practice: we will suggest optimum seed mixtures to be used by farmers. Although the project is focussed on N2O emissions, we will evaluate the overall effect of our plant combinations on N use efficiency, which is a key issue of interest for farmers that addresses concurrently aspects of environmental and economic sustainability. Prior studies on N2O emissions have measured biogeochemical processes but have not considered the underlying bacterial community responsible for these processes in relation to plant functional traits. Microbial ecologists will thus be highly interested in our results, particularly by the new insights regarding plant-trait effects on microbial communities. Our multidisciplinary project will provide a cost-efficient tool to help tackle simultaneously the challenges of food security, biodiversity and combating/adapting climate change.
Greenhouse Experiment
Field Experiment
Showing how to take gas samples