Manipulating plant community composition to steer efficient N‐cycling in intensively managed grasslands

Abstract

Minimizing nitrogen (N) losses and increasing plant N uptake in agroecosystems is a major global challenge. Ecological concepts from (semi)natural grasslands suggest that manipulating plant community composition using plant species with different traits may represent a promising opportunity to face this challenge. Here, we translate these trait‐based concepts to agricultural systems in a field experiment, aiming to reveal the main determinants of how plant community composition regulates N‐cycling in intensively managed grasslands. We focused on key N pools (plant N from soil and from biological N‐fixation, soil mineral N and N2O emissions) as well as on biological drivers of N‐cycling in soil (abundance of N‐cycling microbial communities, earthworm populations and arbuscular mycorrhizal fungi), using three common grass and one legume species in monoculture, two‐ and four‐species mixtures. We hypothesized that: (a) plant species mixtures increase plant N uptake, reduce soil mineral N concentrations and N2O emissions and promote the abundance of biological N‐cyclers; (b) legume presence stimulates N pools, fluxes and biological N‐cycling activity, (c) but in combination with a grass with acquisitive traits, more N is retained in the plant community, while N2O emissions are reduced. We found that mixtures increased plant N and lowered the soil mineral N pool compared to monocultures. However, plant species identity played an overarching role: Legume presence increased N2O emissions, plant N pools, soil mineral N and the abundance of N‐cycling microbes and earthworms. Combining the legume with a grass with low leaf dry matter content and high root length density (and with high root biomass) reduced the higher soil mineral N and N2O emissions induced by the legume, while harnessing positive effects on plant N pools and biological N‐fixation. Synthesis and applications. Our results show the potential of plant community composition to steer N‐cycling in fertilized agroecosystems, paving the way for a more biologically based agriculture. Legumes will play a crucial role, but selecting an optimum companion species is key for the sustainability of the agroecosystem.

Plant traits of grass and legume species for flood resilience and N2O mitigation

“…Acquisitive grasses also had higher recovery 5-week post-flood, which was related to lower N 2 O emissions. This is broadly in line with studies reporting that acquisitive, productive grasses can mitigate N 2 O emissions in non-flooded conditions (Abalos et al, 2018(Abalos et al, , 2020.…”

“…Flooding drastically increased N 2 O emissions, consistent with previous studies Sánchez-Rodríguez et al, 2019 . Legume LDMC was negatively related with N 2 O emissions until the end of the flood potentially also because of slower decomposition (Freschet et al, 2012 (Abalos et al, 2018(Abalos et al, , 2020.…”

Comparison of two image analysis software for root trait analysis of single and mixed species grasslands

“…Improved resource acquisition is a commonly reported benefit of such systems and can be linked to root functional traits such as specific root length (SRL) and root tissue density (RTD), which are calculated from root length and volume, respectively (Li et al., 2014; Rose, 2017; Weisser et al., 2017). These functional traits are often reported on the community level because of difficulties in differentiating between roots of different species (e.g., Albalos et al., 2021; Legay et al., 2014).…”

Stable Nitrogen-Cycling Capacity in Relation to Fertilization and Intercropping in a Sub-Boreal Grassland

“…The potential of NO 2 − reduction by DNRA versus denitrification, estimated as the nrfA/nir gene abundance ratio, has been suggested as a proxy measure for N retention [12]. However, the effects of N fertilization and plants on soil functional communities can vary with fertilizer type, plant species and functional group analyzed [13][14][15][16][17][18]. Most of these studies are based on sampling a single time point, so there is limited information on how fertilizer type and legume-grass intercropping management affect the N-cycling microorganisms over time under field conditions.…”