*Article* **Autumn Tillage Reduces the Effect of Plant Cover on Topsoil Nitrogen Leaching**

**Jorge F. Miranda-Vélez 1,\* and Iris Vogeler 1,2**


**Abstract:** Keeping cover crops to reduce nitrogen leaching often conflicts with timing tillage operations before the soil becomes un-trafficable during winter, while leaving cover crops in the field until spring raises concerns over pre-emptive competition with the following crop. Therefore, farmers may resort to tilling their fields in autumn after letting cover crops remain in the fields for only a short period of time. We explore the effects of this practice in a laboratory lysimeter setting by analyzing the leaching of nitrate from intact topsoil cores. Cores were extracted from no-till (NT) plots and plots tilled in autumn (AuT), in areas kept bare (B) and with volunteer winter rye plant cover (V) after harvest. Nitrate breakthrough curves show that V significantly reduced N leaching by 61% relative to B in NT, but did not have a significant effect in AuT. Dissection of leached cores and undisturbed reference cores indicated a significant removal of mineral N from the soil during the lysimeter experiment for all treatments except V in NT. This indicates that volunteer cover removed a crucial amount of leachable N and suggests that tillage counteracted the effect of V in AuT, likely due to a combination of reduced uptake and re-mineralization of N in cover crop residue.

**Keywords:** nitrogen; nitrogen leaching; autumn tillage; no-till; lysimeter

**1. Introduction**

Keeping plant cover between harvest and the sowing of the subsequent cash crop, i.e., cover crops, has many advantages in agriculture. Cover crops can be used as a natural control against weeds [1] and to reduce nitrogen leaching after harvest [2,3]. Additionally, cover crops are known to protect agricultural soil from erosion, improve soil structure and fertility, and stimulate carbon storage in the soil [4–6].

Concerns over pre-emptive competition over nutrients [7] and water [8], as well as poor field trafficability in early spring and autumn spreading of animal manures [9], often motivate farmers to limit the residence time of winter cover crops. Thus, cover crops successfully established after harvest may be terminated in the same autumn, depending on the farmer's judgement. In the Nordic countries, for instance, the use of post-harvest cover crops is either compulsory with area requirements depending on management (Denmark) or is encouraged and subsidized (Sweden, Norway and Finland). However, farmers are typically free to till their fields and terminate the cover crops after 20 October in Denmark and Sweden and after 1 October in Finland. Only in Norway is the associated subsidy contingent on spring termination of the cover crops [10].

Cover crops in temperate climates extend roots and take up soil N during a relatively short growth period before being killed by frost or terminated by the farmer. During this period, however, post-harvest N uptake by cover crops in temperate climates frequently represents over 20% of the applied fertilizer N, resulting in a 35% average reduction in soil mineral N in autumn and reducing nitrate leaching by 40 to 70% [11,12]. Upon cover crop termination, litter and root breakdown in the soil results in a gradual re-release of a significant fraction of the taken-up N. The timing and extent of this process is affected

**Citation:** Miranda-Vélez, J.F.; Vogeler, I. Autumn Tillage Reduces the Effect of Plant Cover on Topsoil Nitrogen Leaching. *Nitrogen* **2022**, *3*, 186–196. https://doi.org/10.3390/ nitrogen3020014

Academic Editor: Jacynthe Dessureault-Rompré

Received: 14 March 2022 Accepted: 11 April 2022 Published: 13 April 2022

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by a number of factors—e.g., type of cover crop, degree of incorporation, air and soil temperature, precipitation, and termination time—and is key to the overall effect of the cover crops [13]. If a significant amount of soil N is taken up by the cover crops and remineralized during the following growth season, it will be available for uptake by the cash crop and less fertilizer need to be applied to the soil. Indeed, Danish regulations require a fertilizer reduction of up to 25 kg N ha−<sup>1</sup> following cover crops in order to utilize the N "carried over" by the cover crops [10]. However, if the time for soil N uptake by cover crops is short and/or re-mineralization of cover crop N takes place during a period of fallow with high precipitation, it will be at high risk of leaching [14,15]. These considerations, unfortunately, often conflict with farmer's habits and their concerns involving weed control (including the cover crop) and reduced biomass and grain yields on the following cash crop [10,16,17].

This study explores the effect of autumn tillage on the capacity of cover crops to reduce nitrogen leaching. We compared the effect of autumn inversion tillage (AuT) against no-till (NT) on the reduction of nitrogen leaching by volunteer winter rye as cover crops in a laboratory lysimeter setting. We expect that the effectiveness of the volunteer cover crops will be diminished by autumn tillage for two reasons: (1) shortening of the time available for the volunteers to take up N from the soil and (2) re-mineralization of nitrogen in the incorporated volunteer residue. Thus, we hypothesize that soil mobile N contents will be high in AuT regardless of cover crop treatment, and that leached N recovery from AuT cores with volunteer cover crops will be greater than in corresponding NT cores.
