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INPLAMINT - Increasing agricultural nutrient-use efficiency by optimizing plant-soil-microorganism interactions

Subproject F

Dr. Kathleen Lemanski and Prof. Michael Bonkowski

Terrestrial Ecology Group, Institute of Zoology, Department of Biology

Universität zu Köln

Intensive agricultural practices lead to nutrient imbalances, which are the cause of detrimental nutrient losses, groundwater pollution and greenhouse gas emissions. In order to increase nutrient efficiency and maintaining crop productivity in a sustainable way subproject F is aiming to optimize the stoichiometric coupling of the relative sizes of nutrient pools (C:N:P) in soils, microorganisms and crop plants, and the respective nutrient dynamics during uptake and release.

Microorganisms are the ultimate drivers of the bio-geo-chemical cycles of carbon (C), nitrogen (N), and phosphorus (P) in soil. The composition of soils, microorganism and plants is characterized by different stoichiometric element ratios. Microbial processes narrow the C:N:P ratio during litter decomposition and the built-up of soil organic matter. The relative availability of C:N:P determines if a particular nutrient will be released for plant uptake or locked up in the soil microbial biomass. Nutrient losses result from element imbalances and wrong timing of microbial mineralization and plant uptake.

Subproject H aims to characterize and to manipulate stoichiometric threshold ratios in order to

  • reduce soil nutrient losses by improving the timing of microbial mineralization and plant nutrient uptake
  • characterize nutrient threshold ratios to precisely predict crop nutrient demand; and to improve shoot-to-root allocation in crop plants in dependence of mycorrhizal symbionts
  • give recommendations for an optimized restoration and sustainable long-term management of soil fertility in agricultural soils.

Overview of main study sites and experiments:
Shifts in soil and microbial nutrient pools and enzyme activities in a post-mining chronosequence
The restoration of agricultural soils after open-cast mining offers a unique opportunity to study shifts in the nutrient balance during the development and over the long-term management of agricultural soils. Our study sites are located along a soil restoration chronosequence following an open cast lignite mining (RWE Power, Germany), approx. 50 km west of Cologne, Germany. We investigate changes in the stoichiometric balance of soils and microorganisms over 50 years of agricultural practice after soil restoration. Extracellular enzymes in soil are the direct functional expression of microbial activities. We measure shifts in enzyme activities as indicators of microbial nutrient demand to characterize the phases of soil development during restoration and soil degradation during intensive agricultural management.

Medicago sativaFig. 1: Alfalfa (Medicago sativa) increase water drainage by deep-rooting tap roots, and are grown to increase the soil Norg contents on a new reclaimed

Microbial nutrient limitation as predictor of crop yield in a long term field experiment

Due to the slow build-up and inherent resilience of soil nutrient pools, only long-term field experiments provide enough sensitivity to reveal changes in soil functioning. We study different measures of microbial nutrient limitation as predictors of future crop yield in a long-term experiment where soil nutrient pools have been manipulated by organic and mineral fertilization and intercropping with catch crops. We hypothesize that microorganisms and plants compete for the same soil nutrients, and therefore microbial nutrient limitation can be used as sensitive predictor of the current crop nutrient demands.

Stoichiometric nutrient threshold ratios determine mycorrhizal functioning and shoot-to-root allocation in barley (Greenhouse Experiment)

In a greenhouse experiment we examine the influence of the relative availability of N and P on shifts in the allocation of resources of barley plants to shoots and roots that are either colonized by arbuscular mycorrhizal fungal symbionts or not (Fig. 2). We aim to characterize nutrient ratios under which plant investments shifts from symbionts to roots and to shoots. Mycorrhizal fungi are expected to change their symbiotic nature from mutualism to parasitism under increasing nutrient availability in soils, so we are interested in tipping points and stoichiometric threshold ratios when the symbiosis will change.

University of CologneFig. 2: Pot experiment with barley plants grown at different N and P fertilizations treatments.