Chemically oxidized biochar increases ammonium-15N recovery and phosphorus uptake in a grassland

Abstract

Soil amendment with biochar is shown to be a low-cost carbon sequestration option while its properties contribute to nutrient dynamics. The properties of biochar can be purposefully modified using different techniques including chemical oxidation. A change in biochar functionality, to an equivalent state of natural aging, could alter plant nutrient use efficiency, particularly nitrogen (N) and phosphorus (P), which however has not been examined yet. Here, we examined 15N recovery and P uptake in mixtures of clover (Trifolium repens) and ryegrass (Lolium perenne) after biochar amendment at different oxidation states (i.e., fresh, 5%OBC and 15%OBC, oxidized with 5 and 15% H2O2, respectively) and addition of 15NH4+-N and 15NO3−-N. Overall, after 30 days of growth, the total 15N recovery in plant and soil was 41% greater with NH4+-N than with NO3−-N addition. However, the 15N recovery from NH4+-N addition was greater in the oxidized biochar treatments than the control and fresh biochar treatments (a significant biochar × N interaction). Chemical oxidation also increased leaching of NO3−-N and phosphate-P while greater plant P uptake occurred in the 15%OBC treatment with NH4+-N addition. An increase in cation exchange sites with oxidized biochar may have increased NH4+-N retention and biomass production (N uptake) and, thus, 15N recovery. The increase in P availability may further have increased plant biomass production in the 15%OBC treatment with NH4+-N. Our results suggest that oxidized biochar could increase primary productivity by increasing NH4+-N use efficiency and P availability while increasing the risk of NO3−-N and phosphate-P leaching.