Experimental study on the gross N dynamics in the agricultural fields of different land use history

Background/Question/Methods

Processes of soil N cycling are key functions in determining N availability and losses in terrestrial ecosystems. In agricultural fields, management practices greatly affect the sustainability of the system. Here we show for the first time differences in N cycling processes in situ between two different management practices: 1) traditional ploughing in autumn, resulting in fast decomposition of weeds and straw buried in soil, and 2) no-tillage practice, which leaves all weeds and straw on the ground, and the soil is left unaltered. The latter one has several advantages as compared to the ploughing, i.e. no-tillage decreases soil erosion and P losses, reduces CO₂ emissions and improves carbon content in soil. In 2010 gross N transformation rates under field conditions in the experimental fields of MTT Agrifood Research Finland were quantified by using ¹⁵N tracing principles. After injecting enriched ammonium and nitrate solutions separately into carefully marked soil spots (Staelens et al., 2012), samples were collected at certain time steps within five days, analyzed for ammonium and nitrate concentrations and their isotope enrichment. For quantification of gross N transformations a numerical tracing model is used (Müller et al 2007).

Results/Conclusions

Within the first time interval (24 hours from the ¹⁵N injection), in the traditionally ploughed soil ammonium mineralization was about 67 % of that in the no-tillage soil (0.030 versus 0.045 µg ammonium-N g^-1 dw h^-1). Also immobilization showed the same trend (0,022versus 0.034 µg ammonium-N g^-1 dw h^-1). Nitrification and nitrate immobilization rates were about one tenth of the rates for ammonium. They showed similar trends than the rates for ammonium, i.e. smaller values in the traditionally ploughed than in no-tillage soil (0.0024 versus 0.0051 for nitrification and 0.0022 versus 0.0046 for nitrate immobilization). Using modelling (Müller et al 2007), we will determine more parameters of nitrogen cycling, but already these preliminary calculations indicate that the technique increasing the amount of water and organic matter in the soil also intensifies soil microbial processes.

 References:

Müller C, Rütting T; Kattge J, Laughlin RJ, Stevens RJ (2007) Estimation of parameters in complex ¹⁵N tracing models by Monte Carlo sampling. Soil Biology & Biochemistry 39, 715-726.

Staelens J, Rütting T, Huygens D, De Schrijver A, Müller C, Verheyen K, Boeckx P (2012) In situ gross nitrogen transformations differ between temperate deciduous and coniferous forest soils. Biogeochemistry 108, 259-277.