Linking land use intensity, biodiversity, soil microbial processes and organo-mineral interactions for a mechanistic understanding of full nitrogen balances in grassland ecosystems

Background

Increasing land use intensity to produce more protein-rich fodder might be directly linked with detrimental effects on biodiversity and multifunctionality of grassland ecosystems and drive environmental nitrogen (N) losses. Understanding the effects of LUI on soil organic nitrogen (SON) and soil organic carbon (SOC) changes, and reciprocal interactions with microbial and plant biodiversity, therefore requires a hitherto unavailable mechanistic understanding of entire grassland ecosystem N balances, covering all N inputs, internal N transformation processes, and N losses. While the role of plant N exports in this balance is relatively well known, current key uncertainties are (1) total fertilizer gaseous N losses including not only ammonia emissions but also dinitrogen losses from denitrification (2) the drivers of the balance of fertilizer N retention in SON pools vs. the release of N from SON pools by mineralization, and (3) quantities and drivers of N inputs by biological nitrogen fixation depending on LUI. In BioMON II we use the ¹⁵N tracing experiment we previously established at all exploratories as a unique opportunity to unlock these unknowns as a basis for mitigating environmental impacts of grassland management.

Goals

Our overarching goal is to provide a mechanistic understanding of grassland N turnover by providing full grassland N balances as driven by LUI, plant- as well as microbial abundance and diversity, climate, soil properties and SON dynamics. More specifically, our detailed objectives are:

O1: To assess effects of LUI, climate, soil properties and OM stabilization mechanisms, plant diversity and diversity of microbes catalyzing major steps of N transformation on five-year fertilizer N balances, thereby providing insights into retention and turnover mechanisms of SON pools, plant uptake, and N loss quantities and pathways (WP1).
O2.1: To assess effects of grassland extensification on legumes-soil-microbe interactions that govern the rates of BNF and the size of associated SON retention pools (WP2.1).
O2.2: To better understand the rates and fates (as related to OM dynamics of detritusphere vs. rhizosphere) of BNF-N under high and low LUI, the seasonal dynamics of BNF rates, its relations to microbial diversity, and activity of symbiotic and free-living diazotrophs (WP2.2).
O3: To synthesize project measurements across both phases with BE data to uncover the mechanisms that drive changes of full ecosystem N balances under different LUI and biodiversity (WP3).

Hypotheses

H1: The mean residence time of fertilizer-N in SON is a key driver of plant N uptake and ecosystem N balances, with short mean residence times under high LUI as related with higher microbial activity and mineralization at smaller C:N ratios leading to high productivity and mining of SON pools.

H2.1: Short-term extensification of a few years will not increase BNF as mineralization of SON is leading to continuously high N availability and productivity. Shifts in nodule community composition and SON pool size in POM are early indicators of extensification effects (WP2.1).

H2.2: Under long-term low LUI, BNF significantly contributes to positive soil N balances due to high rates of N inputs. Mowing vs. mulching is deciding on loss of BNF-N versus detrital stabilization in SOM pools. At high LUI, reduced BNF efficiency of legumes is partly compensated by a shift from symbiotic to free-living N₂ fixing bacteria, accompanied by a shift from truncated denitrification (symbiotic N fixers) to full denitrification (free-living N fixers), which decreases the ratio of N₂ emissions over N₂O emissions (WP 2.2).

H3: The underlying mechanisms explaining the N balance differ across LUI, soil properties, and regions. Thereby microbial activity measurements linked to process rates and differentiated SON pools are better predictors than microbial abundance, total N, and soil type (WP3).

Methods

In this project, we follow the fate of organic fertilizer N in the plant-soil-microbe system using 15N stable isotope tracing approaches at selected plots of all three Biodiversity Exploratories. For this, we trace different fertilizer N sources (farmyard manure, liquid slurry) and BNF-N and their transformation by biotic and abiotic processes to assess their partitioning towards plant N uptake, gaseous and hydrological N loss pathways as well as microbial N retention and subsequent storage in and release from ON pools up to five years. This includes the analysis of the dynamics of abundance, diversity and activity of microbes which catalyze major steps of the transformation of organic and inorganic N compounds, and of the N residence time in different ON pools.

Our novel Nitrogen (N) cycling framework integrates biological, mineral, and organic N dynamics in agricultural soils under different land use intensity. This allows disentangling management-related shifts of N binding as (a) occluded particulate organic matter and at mineral surfaces, (b) microbial N retention as a microbial biomass “N pump”, as well as (c) biological N fixation depending on the activity of symbiotic, free-living or associative bacteria. WP 1, 2, 3: key uncertainties addressed by respective WP in Biomon II.

Results

Results and Conclusion from BE_BioMON phase I

The mechanistic insights obtained from 15N fertilizer tracing at all exploratories lead us to develop a new paradigm for grassland N cycling:

In organically fertilized grasslands, the soil is fertilized, not the plant: Only ca 10% of fertilizer N is taken up by plants, with the rest either being immobilized by soil microbes for subsequently stabilization in soil organic matter, or being lost along gaseous pathways.
Therefore, the majority of plant N uptake (>90%) is not from recent fertilizer but mainly from mineralized soil organic nitrogen.
Also the abundance and activity of soil nitrifiers is strongly depending on soil organic nitrogen properties, specifically on mineral associated organic nitrogen
Gaseous N losses after organic fertilization strongly increase with land use intensity at all three exploratories, while extensive management with farmyard manure largely prevents environmental N losses.
The risk for soil organic nitrogen mining increases with decreasing soil C:N ratio. Specifically for soils with a topsoil C:N ratio <11 we therefore recommend particularly efficient nitrogen management.

Furthermore, we constrained the potential of bulk soil δ¹⁵N as an indicator of previous management history. By analyzing δ¹⁵N changes between 2011 and 2023 at all exploratories, we could distinguish isotopic fingerprints of extensive management (+0.1±0.2 ‰ change of δ¹⁵N per decade) vs intensive management (+0.5±0.1 ‰ change of δ¹⁵N per decade). Given these slow changes, δ¹⁵N is an integrative indicator of the long-term management history of several decades rather than years. Interestingly, at all three exploratories SON concentrations decreased at high but not at low LUI, with increasing δ¹⁵N values under high LUI reflecting N losses.