Carbon input : Soil carbon input through crop plants

Soil organic matter is the key factor for ecosystem services provided by soils, such as soil fertility, water quality, protection against erosion, and climate mitigation through carbon sequestration. The maintenance or increase of soil organic matter through plant-derived inputs is a central goal of soil protection strategies in agriculture. However, our understanding is very limited of how carbon inputs vary with management practices, crop species, or even with different crop varieties under current and future climate conditions.

The project aimed to quantify the below ground carbon inputs by arable crops as the main source of soil organic matter formation. Further investigations shed light on dependencies of below ground carbon inputs on agricultural management practices, genetic crop factors and drought events for Switzerland. The results will be implemented in carbon models for application in Switzerland’s greenhouse gas inventory and agro-environmental indicators. In addition, they can be used as a base in practical tools for calculating carbon balances and life-cycle analyses used by advisory services and farmers.

Intensive agriculture, in particular fertilisation does not affect below ground carbon inputs (roots and rhizodeposition) of maize, wheat and rapeseed. However, due to a strong increase of above ground carbon the ratio of below to above ground carbon decreases significantly with increasing management intensity. Hence, below ground carbon inputs cannot be estimated by a simple constant factor based on above ground yield parameters. The data provide a basis for improving soil carbon models with the aim of minimising the models’ complexity in relation to general data availability, e.g. for crop yields.

Modern dwarf wheat varieties only dig deep if they need to: when compared to old, tall varieties, modern, semi-dwarf wheat varieties have reduced rooting depth but can adjust in case of drought.

The team compiled a comprehensive field data set of below ground carbon inputs in relation to agricultural management for several crops, cultivars and sites –larger and more detailed than anything currently available.

Breeding appeared to decrease carbon inputs into soil in a similar way as it decreased the above ground biomass to maximise the harvest. The lower carbon input is not only a result of less root biomass but also reduced respiration rates.

The project shows the effects of agricultural management on below ground carbon inputs by arable crops and contributes to improving soil carbon models and applied carbon balances. The implementation of a carbon balance tool in the new version of the Swiss fertilisation recommendations (GRUD 2016) is currently under discussion.

If different reduced height genes could be utilised to adjust rooting depth, this would be a benefit to breeders. The team is currently generating a mapping population to map the genomic regions controlling these differences.

Agricultural management and below ground carbon inputs – Sustaining soil quality