Soil and Water Management: Practices to Mitigate Nutrient Losses in Agricultural Watersheds

1. Introduction

As the world’s population grows, the demand for food is increasing as never before. In this scenario, the sustainable development of agricultural watersheds is an important foundation of food security [1]. Nutrient losses from agricultural watersheds lead to losses of nutrients (e.g., nitrogen, phosphorus, potassium, etc.) from the soil due to runoff or a variety of other reasons, and are considered to be one of the most serious threats to farmland. Many studies have proven that nutrient losses from farmland may cause a decrease in soil fertility that affects crop growth and development, which leads to crop yield reduction [2]. Nutrients lost in the soil, especially nitrogen and phosphorus, may enter water bodies through surface runoff or erosion, leading to eutrophication and damage to water ecosystems. In addition, the loss of nutrients from agricultural land can lead to a number of problems such as soil salinization, changes in soil microbial communities, increased soil erosion, and soil compaction. Therefore, it is crucial to apply soil and water management to reduce nutrient loss from agricultural land and maintain sustainable agriculture [3].

Practices that can be used for mitigating nutrient losses in agricultural watersheds include conservation tillage, cover crops, precision agriculture, diversified cropping systems, etc. These practices have been proven to have a significant impact on the physical, chemical, and biological properties of soil, such as soil organic matter content, nutrient availability, microbial biomass and diversity, and enzyme activities [4]. Conservation tillage practices have the effects of reducing soil erosion, maintaining soil structure, and enhancing soil nutrient contents [5]. The incorporation of cover crops into soils contributes to the enrichment of soil nutrients, robust nutrient cycling processes, and the improvement of nutrient availability [6]. Precision agriculture can effectively avoid the risk of over-applying chemical fertilizers, which can lead to a large amount of nitrogen and phosphorus escaping from farmland into the atmosphere and water bodies, exacerbating the risk of environmental pollution [7]. Diversified cropping systems can improve crop coverage to mitigate the scouring effects of rainfall, which in turn reduces runoff and thus nutrient losses in agricultural watersheds. In addition, diversified cropping systems can improve soil fertility, control insect pests and diseases, and help in attaining yield stability [8]. Choosing the appropriate soil and water management practices for different nutrient losses requires clarification of the transport patterns of nutrients lost on farmland and the damages caused by nutrient losses. The rising prevalence of these issues led to the creation of this Special Issue, entitled “Soil and Water Management: Practices to Mitigate Nutrient Losses in Agricultural Watersheds”. This Special Issue aims to explore the following topics: (1) Mechanisms of nutrient transport in agricultural watersheds. (2) Methods for the quantitative assessment of nutrient losses in agricultural watersheds. (3) Damages caused by nutrient loss in agricultural watersheds. (4) Practices that can be used for mitigating nutrient losses in agricultural watersheds, including conservation tillage, cover crops, precision agriculture, diversified cropping systems, etc.

2. Summary of the Special Issue

We selected five original research papers after a rigorous peer-review process. The below section outlines the list of contributions.

Contribution 1 assesses the intra- and extracellular metabolism of Arsenic (As), along with speciation changes, in Microcystis aeruginosa across three growth phases. The findings of this study revealed that extracellular iAs^V remained the dominant As species during the lag and exponential growth phases of M. aeruginosa in the growth media, while intracellular trivalent As (iAs^III) emerged as the pronounced species during the exponential growth phase, but also exhibited a significant negative correlation with the P levels. Moreover, elevated P levels promoted the formation of intra- and extracellular dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA) in the exponential growth phase. During the stationary growth phase, intracellular iAs^V was found to decrease with increasing P levels. During all of the growth phases, P consistently reduced algal As absorption levels.

Contribution 2 used computational, experimental, and field methods to design and evaluate alternative tile intakes (ATIs)’ capacity to reduce sediment and nutrient export. The results demonstrated that the hydraulic conductivity of the layered gravel–woodchip configuration was 4.59 ± 0.36 cm/s. The sediment trapping efficiency of the ATIs was 86 ± 12%, leading to deposition rates of 5.44 ± 3.77 cm/yr, as quantified with 210 Pb profiles. Modeling using the Agricultural Conservation Planning Framework suggested watershed-scale load reductions of 1.6% for NO₃ and 1.4% for total P for ATIs draining 6.8% of the modeled watershed.

Contribution 3 investigated the influencing factors of heavy metals and non-point source pollution in typical areas of the Tethys Himalayan Tectonic Domain by analyzing 44 water samples and 55 soil samples. The results indicated that 65.46% of soil As in the study area exceeded the screening value, while the concentrations of the eight selected elements in water remained below the standard limits. Factors such as the per capita net income, mean annual temperature, mean annual precipitation, geomorphic type, organic matter, geology type, and soil texture (sand, silt, and clay) constituted the primary controlling factors influencing the spatial distribution of HMs in soil.

Contribution 4 constructed a hydrographic connectivity evaluation system for Henan Province, China, spanning the preceding two decades (2000–2020). The results revealed that in terms of structural connectivity, agricultural land accounted for over 50% and prevailed as the primary land-use type; reservoir and lake areas initially increased before subsequently decreasing. Human activities have exerted a profound influence on these changes. Meanwhile, the structural form of the water system has gradually improved, exhibiting an increasing complexity of river networks and a stabilizing connectivity configuration. As for functional connectivity, the natural function remains well preserved, while the social function demonstrates a positive correlation with the expansion of industrial activities, eventually growing to an excellent level from an initial moderate level.

Contribution 5 collected overlying water and sediment samples for two typical agricultural ditches in Sanjiang Plain during the growing seasons of 2015–2017 to clarify the interception of internal and external nitrogen in ecological ditches. The results indicated that the N-NO₃⁻ in the overlying water was higher than N-NH₄⁺, while in the sediment, N-NH₄⁺ was higher than N-NO₃⁻. In contrast to the dryland ditch, the paddy ditch had a more significant amount of inorganic nitrogen both in the overlying water and sediment. In both the overlying water and the sediment of the ditches, nitrogen content fluctuated during different periods, and inter-annual variation was noticeable.