Exploring the effects of soil structure, nutrients, and farm management on crop root biomass and depth distribution

Context

Roots are important for crop productivity and soil C sequestration but are difficult to study because they are hidden underground. Consequently, they are often estimated from aboveground biomass, ignoring the variability caused by belowground soil properties and farm management.

Objective

Our study aimed to identify the main factors that explain the variability in root biomass encountered on working farms.

Methods

We investigated 20 farms during two years (2021 and 2023), across soil and crop types, soil fertility ranges, and farming systems. We measured root biomass from collected samples at 10-cm intervals from soil surface to a depth of 60–100 cm. Root biomass and depth distribution were compared with auxiliary data on soil properties and farm management.

Results

We found that aboveground biomass was a poor predictor of root biomass and that root:shoot ratios were highly variable. Root biomass was controlled by a combination of soil texture, structure, nutrient availability, and management history. The main determinants of root biomass and depth distribution differ between annual and perennial crops. In addition, the topsoil 0–20 cm contains only 60–70 % of the crop roots, highlighting the importance of also investigating roots in the subsoil.

Conclusions

Soil texture, structure, and nutrients determine root biomass to a greater extent than crop type (perennial vs. annual) or aboveground biomass. They should be considered in more detail when estimating root biomass and when exploring ways to increase it.

Implications

Our findings highlight the importance of mitigating soil compaction, maintaining adequate but not excessive crop nutrition, and managing soil water, also for growing larger crop root systems.