Barley–pea intercropping: Effects on land productivity, carbon and nitrogen transformations

•Intercropping increased land productivity, C and N accumulation over monoculture.•Pea intercrops improved nodulation and %N derived from symbiotic N2 fixation.•1:1 arrangement displayed the highest N transfer over 2:1 or mixed arrangements.•2:1 sequestered the most C compared to other planting arrangements.•Cropland absorbed the most CO2 during mid-growth stage (i.e., prior to flowering).

Declining land productivity associated with decreasing soil organic carbon (SOC) and nitrogen (N) are significant issues in monoculture barley production. An intercropping system combining barley and pea may help increase land productivity as well as to maintain SOC and soil mineral N. We grew barley as a monoculture and intercropped with pea without fertilizers in rows of barley:pea 1:1 and 2:1 as well as broadcast arrangements for two years to observe the effects of species ratios and spatial configuration on land productivity, biological nitrogen fixation and transfer, C and N accumulation in aboveground biomass, soil mineral N balance, gross ecosystem photosynthesis (GEP), and net ecosystem productivity (NEP). 15N natural abundance method was used to quantify N fixation and subsequent transfer. Field CO2 exchange measurements used a dynamic closed transparent chamber connected to a portable CO2 analyzer.Intercropping displayed higher land productivity (12–32%) compared to monoculture plots, with 2:1 arrangement producing the highest total land outputs – TLO (5.9 t ha−1) and land equivalent ratio – LER (1.32) values. Intercropped barley showed higher biomass N, grain protein and sequestered higher C in soil compared to monocultured barley. Intercropped pea displayed increased nodulation (27–45%) and symbiotic N2 fixation (9–17%) compared with monoculture pea resulting in the fixation of 60–78 kg N ha−1. The highest rate of N-transfer (11%), and increased N accumulation (i.e., 200% higher than monocultured barley) in shoot biomass was observed in 1:1 arrangement. However, 2:1 arrangement accumulated higher C (196 g C m−2 year−1, i.e., 53% higher) in shoot biomass compared to the monoculture barley plots. The 2:1 arrangement also displayed the greatest NEP resulting in the highest soil C sequestration at a seasonal daytime average rate of 229 mg C m−2 h−1 (i.e., 10% higher than barley monoculture plots). This study demonstrated that intercropping barley and pea is an efficient strategy to increase land productivity, grain and biomass quality, N and C yields, GEP and NEP, and that planting in rows of 2:1 was the most productive arrangement.