Why enhanced nitrogen and phosphorus use efficiency?
Appropriate crop nutrient management is key to achieve maximal yields and good yield quality but is challenging in many ways. Nitrogen is relatively easily available for plants, but has high risk of being lost via nitrate (NO3–) leaching or N2O emissions. This leads to inefficient nitrogen use, negatively affecting farmer’s economy and the environment, and contribute to climate change. Phosphorus on the other hand is not as easily available as it is easily absorbed by soil particles and organic matter, making it unavailable to plants. Phosphorus is mainly lost by runoff and contributes to eutrophication of water bodies and our oceans.
Biological nitrification inhibition
In brief…
Biological nitrification inhibition (BNI) is the reduction of microbial nitrification posed by biological compounds. The BNIs reduce the amount of NH4+/NH3+ that is oxidised to NO2– and later on to NO3–. These compounds can be produced by plants and either be exudated from the roots during plant growth or released as plant biomass is decaying. It is hypothesised that plants have developed this function to better cope with nitrogen limiting conditions.
BNI in Plantago lanceolata
The compounds that at the moment are considered to be BNIs are aucubin, catalpol and verbascoside, but their individual contribution still needs to be verified. Plantago lanceolata, or plantain, is a small herb that can be used in leys and grasslands as well as in an arable setting. In livestock system plantain could have multiple benefits, apart from reducing N losses from soils it has also proven to reduce the amount of excreted N (or at least the concentration) and it also has some anti-bacterial properties in the gut and intestine of ruminants. In our experiments we will study in depth the dynamics of N in different systems to which plantain has been added, to better understand to what extent we can see effects in field and when the BNI capacity is the highest.
A more mechanistic explanation
The compounds involved in BNI inhibits the production of enzymes active in the oxidation of ammonia (ammonia monooxygenase (AMO), see figure) and hydroxylamine (hydroxylamine oxidoreductase (HAO)). However, not all compounds inhibit both oxidation steps. As a comparison, synthetic nitrification inhibition (SNI) only affects AMO genes. In screening studies, BNI from different plants has shown as high nitrification inhibition as SNI.
Nitrification makes nitrogen more prone to losses, either as leached NO3– or gaseous losses of NO and N2O. Nitrate leaching contribute to eutrophication while N2O emissions contributes to global warming. Reducing these losses of N is hence of great societal importance.
Many plants favour uptake of NO3– over NH4+. Understanding how BNI crops can be used in cropping systems to reduce losses without reducing crop yields is therefore important for optimal use of these crops.
Phosphorus mobilisation
In brief…
The limited availability of phosphorus (P) in agricultural soils is a key challenge for sustainable crop production, as phosphorus is essential for plant growth and development. Phosphorus often exists in soils in forms that plants cannot easily absorb, leading to a deficiency despite its abundance in the soil. Plants have developed several mechanisms to increase phosphorus availability.
Phosphorus mobilising service crops
Plants have developed several mechanisms to increase phosphorus availability. One primary method involves the release of organic acids, such as citric and malic acids, from their roots. These acids help dissolve bound forms of phosphorus, converting them into forms that plants can absorb. Additionally, many plants release protons (H⁺) into the soil through their roots, acidifying the rhizosphere. This acidification can displace phosphorus bound to soil particles, making it more accessible. Another mechanism involves root exudates that release enzymes, like phosphatases, which break down organic phosphorus compounds in the soil, freeing up phosphorus in a form that roots can absorb. Some plants even establish symbiotic relationships with mycorrhizal fungi, which extend the root’s reach, allowing the plant to access phosphorus from further away and in lower concentrations. In the FERTIGO project, we will focus on 3 plant species known to mobilize soil phosphorus thanks to their rhizospheric activity: red fescue, Lolium spp. and phacelia.