Real-time monitoring of rhizosphere nitrate fluctuations under crops following defoliation

BACKGROUND: Management regime can hugely influence the efficiency of crop production but measuring real-time below-ground responses is difficult. The combination of fertiliser application and mowing or grazing may have a major impact on roots and on the soil nutrient profile and leaching. RESULTS: A novel approach was developed using low-cost ion-selective sensors to track nitrate (NO(3)(−)) movement through soil column profiles sown with the forage crops, Lolium perenne and Medicago sativa. Applications of fertiliser, defoliation of crops and intercropping of the grass and the legume were tested. Sensor measurements were compared with conventional testing of lysimeter and leachate samples. There was little leaching of NO(3)(−) through soil profiles with current management practices, as monitored by both methods. After defoliation, the measurements detected a striking increase in soil NO(3)(−) in the middle of the column where the greatest density of roots was found. This phenomenon was not detected when no NO(3)(−) was applied, and when there was no defoliation, or during intercropping with Medicago. CONCLUSION: Mowing or grazing may increase rhizodeposition of carbon that stimulates soil mineralization to release NO(3)(−) that is acquired by roots without leaching from the profile. The soil columns and sensors provided a dynamic insight into rhizosphere responses to changes in above-ground management practices.