Identifying resource competition in maize-based soil conservation systems using 13C and 15N isotopic discrimination

Abstract

Soil conservation measures such as establishing grass barriers or cover crops effectively control erosion but also provoke competition, which reduces yields of companion crops. We used 13C and 15N natural abundance profiles to identify the causes of competition of soil conservation measures on a field with 59% slope in Northwest Vietnam three years after establishment. Treatments were maize under farmer’s practice (T1, control), maize with Guinea grass barriers (T2), maize under minimum tillage (MT) with Pinto peanuts as cover crop (T3), and maize under MT and relay cropped with Adzuki beans (T4). A pretest using data from zero-N plots revealed that abundance of water and limited nitrogen availability induced low grain N concentrations, enriched leaf δ13C, and reduced maize grain yield. Similar low N leaf concentrations and elevated δ13C values were observed in maize growing close to frequently pruned grass barriers under positive water balance conditions, indicating that yield decline in these rows can be attributed mainly to N competition. Enriched δ15N values of maize from rows next to barriers indicated reliance on soil N rather than on 15N-depleted fertiliser N. Vigorous cover crop growth under MT resulted in maize yield decline due to N competition while relay-cropped legumes did not trigger inter-species competition having a similar maize yield, leaf N concentration, δ13C, and δ15N as the control.