Does macropore flow in no-till systems bypass mobile soil nitrogen after harvest?

Abstract

Reduced tillage practices, including no-till, have been proposed to reduce NO3- leaching in agricultural soils. However, this is not universally supported by experimental data, and the effect of reduced tillage on NO3- leaching is likely connected to the interaction between macropore flow and resident solutes. To investigate this, we carried out infiltration experiments on intact soil cores from conventionally tilled (CT) and no-till (NT) plots, recording resident concentrations and breakthrough curves (BTCs) of three solutes (a tritium pulse, resident Br- tracer and native NO3-). We then fitted a dual porosity model implemented in HYDRUS-1D, to the experimental BTCs. By assuming different initial solute distributions between phases, we investigate possible bypass mechanisms. In the lysimeter experiment, leaching of all solutes initiated and peaked earlier in NT compared to CT, indicating increased macropore flow. However, total leached NO3- was greater in CT, and higher resident NO3- concentrations were found after the experiment in NT, suggesting an overall bypass effect. In the fitted dual porosity model, saturated water contents in the mobile phase were six times smaller in NT than in CT, with smaller horizontal solute transfer rates for NO3- than for Br-. The optimal initial proportion of total resident NO3- in the mobile phase was 7.5% in NT and 65% in CT. Our results suggest a bypass effect of native resident NO3- in NT soils related to a smaller volume of soil involved in macropore flow. Added solutes, however, remain susceptible to quicker leaching in NT compared to CT under intense precipitation.