water Article Nutrient Load Mitigation with Wintertime Cover as Estimated by the INCA Model Katri Rankinen 1, * , Eila Turtola 2 , Riitta Lemola 2 , Martyn Futter 3 and José Enrique Cano Bernal 1   Citation: Rankinen, K.; Turtola, E.; Lemola, R.; Futter, M.; Cano Bernal, J.E. Nutrient Load Mitigation with Wintertime Cover as Estimated by the INCA Model. Water 2021, 13, 450. https://doi.org/10.3390/w13040450 Academic Editor: Thomas Hein Received: 23 December 2020 Accepted: 3 February 2021 Published: 9 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Biodiversity Centre, Finnish Environment Institute, 00790 Helsinki, Finland; jose.cano-bernal@syke.fi 2 Natural Resources Institute Finland, Natural Resources, Water Quality Impacts, 31600 Jokioinen, Finland; eila.turtola@luke.fi (E.T.); riitta.lemola@luke.fi (R.L.) 3 Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, P.O. Box 7050, 750-07 Uppsala, Sweden; martyn.futter@slu.se * Correspondence: katri.rankinen@syke.fi; Tel.: +358-400-148-832 Abstract: Increased nutrient loading causes deterioration of receiving surface waters in areas of intensive agriculture. While nitrate and particulate phosphorus load can be efficiently controlled by reducing tillage frequency and increasing vegetation cover, many field studies have shown simultaneously increased loading of bioavailable phosphorus. In the latest phase of the Rural Programme of EU agri-environmental measures, the highest potential to reduce the nutrient loading to receiving waters were the maximum limits for fertilization of arable crops and retaining plant cover on fields with, e.g., no-till methods and uncultivated nature management fields. Due to the latter two measures, the area of vegetation cover has increased since 1995, suggesting clear effects on nutrient loading in the catchment scale as well. We modeled the effectiveness of agri-environmental measures to reduce phosphorus and nitrogen loads to waters and additionally tested the performance of the dynamic, process-based INCA-P (Integrated Nutrients in Catchments—Phosphorus) model to simulate P dynamics in an agricultural catchment. We concluded that INCA-P was able to simulate both fast (immediate) and slow (non-immediate) processes that influence P loading from catchments. Based on our model simulations, it was also evident that no-till methods had increased bioavailable P load to receiving waters, even though total P and total N loading were reduced. Keywords: mathematical modeling; agricultural policy; wintertime vegetation cover; dissolved reactive phosphorus 1. Introduction Increased nutrient loading of both phosphorus (P) and nitrogen (N) cause deterioration of receiving surface waters in areas of intensive agriculture. In Finland, agriculture is most intensive and is the dominant land cover on clayey soils situated in the southwestern part of the country from where runoff enters the Baltic Sea. In this region, a major proportion (60–90%) of total phosphorus (TP) load is transported as particulate phosphorus (PP) with eroded soil particles [1], and cultivated fields contribute 66–100% of the total suspended sediments delivered from a small agricultural catchment [2]. Moreover, past and present P excess fertilization is reflected in high soil test p values (STP) of the fields in the area, which is again connected with high dissolved reactive P (DRP) concentrations in runoff from fields [3,4]. Increased P loads, the major nutrient controlling eutrophication in many aquatic systems (e.g., [5]), have been observed to cause eutrophication of surface waters. The Baltic Sea may be seasonally or spatially N limited [6]. Further, P is not always the limiting nutrient in freshwaters. In Finland, some smaller lakes are observed to be partly N limited [7], calling for measures to reduce the loading of both P and N from agriculture. Nitrogen (N) has partly different sources and pathways than P. Most commonly, it is transported as nitrate (NO 3 − ), which is highly leachable and can also percolate into Water 2021, 13, 450. https://doi.org/10.3390/w13040450 https://www.mdpi.com/journal/water