Optimizing soil nitrogen (N) in babyleaf salad crops for sustainable crop production.

The use of synthetic N fertilizers has been responsible for increased food production more than any other input to farming [1]. However, the recovery of fertilizer N in crops is frequently low and fertilizer N not taken up by the crop or immobilized can be lost via soil gaseous N emissions and leaching (Fig. 1). Additional to economic loss to the farmer, environmental damage caused by N losses from cropping systems is significant [2]. According to the ADHB [project FV 370b], the recovery of fertilizer N in babyleaf spinach is only 60% or even as low as 40%.

Our project is therefore motivated by the aspiration to minimize losses of N in babyleaf spinach systems for increased environmental sustainability.

Given that the main N loss pathways initiate from different N forms (urea, nitrate, ammonium; Fig. 1) and under different soil conditions, there exists potential to reduce fertilizer N losses through control of soil N form availability in relation to crop demand and soil conditions. Such control might be achieved via: (i) manipulation of fertilization application form and (split) timing(s), or, (ii) the use of nitrogen cycle inhibitors (NCIs; Fig. 1) or a combination of (i) and (ii). Whilst extensive NCI-related research has been conducted for optimal N use in cereals, there is virtually no information in relation to babyleaf salad systems which differ in many aspects including their short growth cycle (24 to 60 days) which allows a number of crops to be grown in succession within a season with implications for dynamics of soil N availability (and thus fertilizer demand).

The overall aim of this project is therefore to understand N cycle dynamics in babyleaf spinach cropping systems to underpin efficient N fertilization through manipulation of N fertilizer form/timing and NCI addition.

The research will address the following objectives:

1. Examine the interactive effects of N fertilizer formulation, N fertilization timing and NCI addition on:
(a) soil N cycle processes (urea hydrolysis, nitrification, denitrification, mineralization) and their relationship with N loss (as NH3, N2O and NO3-).
(b) babyleaf spinach crop parameters (yield, N uptake, leaf NO3-, NCI uptake, appearance).
2. Examine the extent to which seasonal dynamics of soil N availability and the contribution of soil N to N losses depends on crop rotation and management practices.
3. Evaluate the value of chemical methods for prediction of soil nitrogen supply via mineralization to inform babyleaf spinach fertilizer recommendations.

The PhD student will design, establish and manage a field experiment (on TWC's farmland in Dorset) and also conduct controlled pot experiments and laboratory analysis (University of Reading) using soil and plant material sampled from the field. As indicated in the objectives, treatments will comprise combinations of the following factors: N fertilizer formulation (e.g. calcium ammonium nitrate, ammonium nitrate, urea), rate and timing (e.g. 100% at drilling, 50% at drilling, 50% at first true leaf) and NCI addition (e.g. urease inhibitor nBPT; nitrification inhibitor DCD). In the field, the student will determine NO3- leaching (lysimeters), NH3 volatilization (passive shuttles) and N2O losses (static chambers). Samples will be taken for lab determination of soil mineral N (objective 1a, 2), soil N cycle process potentials (objective 1a, objective 3) and crop parameters (objective 1b). 15N-labelled fertilizer will be applied in pot experiments to examine the relative contributions of soil N and fertilizer N to N loss and plant uptake with 15N:14N ratios of N2O-N, NH3-N (gas samples) and NO3- -N (soil extracts) determined by isotope ratio mass spectrometry (objective 2). Mineralizable N and its relationship with chemical predictors (e.g. organic nitrogen fractions) will be tested (objective 3).