*2.2. Sample Handling and Preparation*

The soil water content of all cores, both Reference and lab-rain, was equalized before the leaching experiment. Each intact core was moved into a 5 ◦C temperature-controlled room and placed on a ceramic plate inside a ~40 L plastic tub and slowly saturated with a simulated soil solution (CaCl2 0.05 M) (Figure 1A). After 7 days of saturation, each core was slowly drained to a pressure of −10 hPa using the same ceramic plates and a controlled vacuum system. Upon reaching equilibrium with the vacuum system, lab-rain cores were transferred to a benchtop lysimeter for a leaching experiment. The Reference cores, in turn, were removed from the ceramic plates upon reaching equilibrium at −10 hPa, and kept lidded at 5 ◦C for the duration of the leaching experiment.

**Figure 1.** Sample preparation (**A**) and leaching experiment (**B**) setup. Preparation consisted of saturating intact soil cores in a simulated soil solution followed by drainage and equilibration at −10 hPa. In the leaching experiment, the soil cores were irrigated with simulated rain at a rate of 10 mm h−<sup>1</sup> for 20 h, while leachate was collected.

## *2.3. Leaching Experiment*

The leaching experiment consisted of 200 mm of simulated precipitation, administered in a single course of 20 h at a constant rate of 10 mm h−<sup>1</sup> on individual benchtop lysimeters (Figure 1B). The lysimeters were fitted with rotating rain heads utilizing blunt needles as dripping nozzles. In order to prevent soil dispersion during the leaching experiment, simulated rainwater consisted of a weak salt solution (1.76 mg L−<sup>1</sup> CaCl2·2H2O + 3.05 mg L−<sup>1</sup> MgCl2·6H2O + 7.07 mg L−<sup>1</sup> NaCl), similar to that used in other leaching studies (e.g., [21,22]). The total simulated precipitation, 200 mm, corresponds approximately to the total precipitation recorded by the meteorological station at AU Flakkebjerg (211 mm) during the months

of October, November and December of 2019. For comparison, the total normal autumn precipitation (September–November, 1961–1990) in Denmark is 228 mm [23]. The leachate from each core was collected in pre-weighed plastic bottles using automatic rotating carrousels at 10-min intervals during the first hour and at 1-h intervals until termination of the experiment.

The nitrate in the collected leachate was quantified by ion chromatography using a Metrosep cation resin suppressor (Metrohm, Switzerland) and an A Supp 5 anion exchange column followed by an electric conductivity detector, with a carbonate buffer as eluent. Leachate samples were filtered using cartridge polyether sulfone (PES) filters (pore size 0.22 μm) before injection.

Following the leaching experiment, both the Reference and lab-rain soil cores were dissected in four layers, corresponding to depths of 0–5 cm, 5–10 cm, 10–15 cm and 15–20 cm, relative to the metal core casings. Each layer in each core was weighed immediately after dissection and placed in an airtight plastic bag, then stored at 2 ◦C awaiting sub-sampling.

Two representative soil sub-samples (~50 g) were taken from each layer in each core. The first set dried at 105 ◦C for 24 h for water content determination at the time of subsampling. The second set of subsamples was weighed and placed in vials for NO3 − and NH4 <sup>+</sup> (mineral N) extraction. Mineral N extraction consisted of suspending the soil samples in 0.2 L of KCl 1M solution and mixing for 30 min in a rotary shaker at 20 rpm, then filtering the supernatant using ashless paper filters. The collected extract was then frozen at −20 ◦C awaiting analysis. Nitrogen as nitrate and as ammonium in the extracts were quantified colorimetrically in a Seal Analytical AA500 auto-analyzer as described by Best [24] and Crooke and Simpson [25], respectively.

### *2.4. Calculations and Statistical Analysis*

All data handling, visualization and analysis was carried out using R version 4.1.0 "Camp Pontanezen", released in May 2021 [26].

Total C, total N and pH measurements were analyzed using linear mixed models with tillage and cover crop treatments as main effects and field block as random effect (packages lme4 and lmerTest [27,28]).

Nitrate leaching (mg) was calculated as the product of the collected volume of leachate at each sampling time and the leachate NO3 − concentration as determined by ion chromatography. The total leached N as NO3 − (NNO3, mg N) was calculated as the sum of the leached NO3 − over the course of the leaching experiment, multiplied by the ratio of the atomic mass of N to the molar mass of the NO3 − ion. The mass of mineral N (Nmin, mg N) extracted from soil subsamples was calculated as the sum of N as NO3 − and NH4 +. Total Nmin was then calculated for each dissection layer and full cores using the wet mass at dissection and the determined water content.

Differences in NNO3 leaching were analyzed using linear mixed models, with total leached NNO3 as response variable, tillage and plant cover as main effects and field block as random effect. Model estimates and 95% confidence intervals for leached NNO3 were obtained from model marginal means (package emmeans [29]) and significant differences were evaluated by multiple pairwise comparison among all tillage and plant cover treatment combinations (package multcomp [30]).

The effects of tillage and plant cover on the differences between the total Nmin content of the Reference and lab-rain cores was evaluated using linear mixed models with experimental group (i.e., Reference or lab-rain), tillage and plant cover as main effects and field block as random effect. Here, all interactions including the three-way interaction were explicitly preserved in the model in order to evaluate the influence of tillage and cover crop treatments on the difference between Reference and lab-rain cores. Model estimates and 95% confidence intervals were obtained from model marginal means and significant differences between Reference and lab-rain were evaluated by multiple pairwise comparisons grouped by tillage and plant cover treatment combinations.

#### **3. Results and Discussion**
