Sustainable Use of Soil Water Resources and Crop High-Quality Production in Water-Limited Regions

Special Article: Soil Science

Ann Agric Crop Sci. 2023; 8(2): 1131.

Sustainable Use of Soil Water Resources and Crop High-Quality Production in Water-Limited Regions

Zhongsheng Guo*

ISWC, Northwestern A & F University, CAS & MWR, Yangling, g, China

*Corresponding author: Professor Zhongsheng Guo ISWC, Northwestern A & F University, CAS & MWR, Yangling, g, China. Tel. ++86-29-87012411; Fax: ++86-29-8701-2210 Email: guozs@ms.iswc.ac.cn; zhongshenguo@sohu.com

Received: March 21, 2023 Accepted: April 28, 2023 Published: May 05, 2023

Abstract

Crop can effectively produce plant productions and services to meet the needs of the people. However, because land use changes alter the plant water relationship, resulting in soil dryness, soil degradation and crop failure in dry years, or waste of soil water resources in wet year in water-scarce areas, both are unfavorable for the sustainable utilization of soil water resources and crops high- quality production. Therefore, it is necessary to adjust the unbalanced plant water relationship, obtain the maximum yield and services to realize the sustainable utilization of soil water resources by plants and crops high-quality management. However, there is not a universally accepted theory to provide guidance of regulation of plant water relationship in practice. Here we show that the theory of sustainable use of soil water resources includes the Soil Water Resources Use Limit by Plants (SWRULP) and Soil Water Vegetation Carrying Capacity (SWVCC). The SWRULP is the soil water resources in the Maximum Infiltration Depth (MID) in which the soil water content in every soil layer equal to wilting coefficient. The wilting coefficient is expressed by the wilting coefficient of indicate plant in the community. When soil water resources in the MID is lower than SWRULP, the plant water relation enters the Critical Period of Plant Soil Relationship Regulation (CPPSRR). The day on which the soil water resources in the MID is lower than SWRULP belong to soil water resources shortage period. If present plant density is more than SWCCV in the critical period of plant soil relationship regulation, plant soil relationship regulation has to be regulated based on SWCCV and then realize sustainable use of soil water resources and get the maximum yield and service. SWVCC is the population or density of indicator plants in a plant community when the soil water supply is equal to soil water consumption in the root zone in the CPPSRR, which changes with plant community type, site condition and time.

Keywords: Soil water resources; Soil desiccation; Soil degradation; Crop failure; Maximum infiltration depth; Soil water resources use limit by plant; Water carrying vegetation capacity; The critical period of plant soil relationship regulation; Sustainable use of soil water resources; High quality production

Significance Statement

Soil is a loose medium and complex system. Soil water is the water existing in the soil space, mainly comes from precipitation and only used by plants in water-limited regions. Because infiltration depth, soil water supply, soil water resources, soil water resources use limit by plant and water carrying vegetation capacity is limit. When the soil water resources in the maximum infiltration depth is smaller than soil water resources use limit by plant, the plant water relation enters the critical period of plant soil relationship regulation, which severely influences plant growth, maximum yield and benefit. At this time, the plant water relationship should be regulated on water carrying vegetation capacity in the critical period of plant soil relationship regulation in case soil degradation, vegetation decline and crop failure to realize sustainable use of soil water resources and crop high-quality sustainable production.

Introduction

In most parts of the world, human activities, such as overgrazing, g, deforestation, denudation and reclamation have greatly changed the type of vegetation that dominates the landscape because the productions and services produced by original forest cannot meet the need of the peoples. Now, w, the man-mad vegetation such as plantation, pasture orchard and crop occupied more than 80% of forest. These have accompanied the demand for food, fruit, timber and biofuels due to local population increases, which historically have frequently occurred in water- limited regions such as the Loess Plateau of China. Intense and poorly managed agricultural practices have often caused a decline in the density of natural plant populations [9,24]. Consequently, the original vegetation has disappeared and there has been a decrease in the level of forest cover and in the ability of forests to maintain a balanced ecosystem. Such changes have led to severe soil and water loss and continual degradation of the natural environment on the Loess Plateau, which severely affects the health and security of forest and vegetation ecosystems and humans.

In order to conserve soil and water and improve the ecological environment and restore harmonious human––nature relationships, since 1950, large-scale afforestation has been carried out on the Loess Plateau. This has been especially supported since 1978 by implementation of projects of the ‘Three-north Protection Forest’ that involves planting trees and establishing protection forest in north-eastern, north- western and northern regions of China to control soil and water loss and improves the ecological environment. Consequently, forest area and degree of cover has dramatically increased, as has the efficiency of forest and vegetation in conserving soil and water. For example, the sediment discharge on the Loess Plateau was reduced from 1.6 billion tons per year in the 1970s to 0.3 billion tons per year in recent years, runoff has halved and the environment has improved.

In the process of vegetation restoration, tree species, selected for their capacity to extend deep roots and for fast growth and conserving soil and water, were planted at high initial planting densities to rapidly establish high degrees of ground cover and higher biomass and yields, and thereby to quickly realize ecological, economic and social benefits in the process of vegetation restoration. It is advantageous that the roots of these plants grow quickly and thus they take up water from considerable soil depths, such as 5.0m for 16-year-old Caragana shrubland (Caragana korshinskii Kom.) in the semiarid loess hilly region in Guyuan County, Ningxia Hui Autonomous Region of China and 22.4m for 23-year-old Caragana (C. microphylla Lam.), another related species in Suide County, Shaanxi Province of China [2]. However, soil water mainly comes from precipitation; and the Maximum Infiltration Depth (MID) and soil water supplies are limited in this region [3]. Thus, root depth can exceed the depth of soil water recharge from rainwater, leading to severe desiccation of soil in root soil layer [35 ,17,18]. Consequently, the combination of increased water use by plants and low water recharge rates has led to soil deterioration after one month or a year or a couple of years, receding vegetation and crop failure on the Loess Plateau in the perennial artificial grass and forest land [3,21,31]. Such soil deterioration can adversely affect ecosystem function and services and the stability of manmade forest and vegetation ecosystems, and consequently reduces the ecological, economic and societal benefits of forest and other plant communities. In turn, this suggests that the Relationship between Plant Growth and Soil Water (RBPGSW) in these perennial artificial grass and forest lands is not harmony and should be regulated.

High-quality managing a man-made ecological system to maximize the yield and benefits of vegetation requires need regulate Soil Water Supply (SWS) and Soil Water Consumption (SWC) over a long time in the water-limited regions. This is because a rapid increase in density and the degree of coverage of forest and/or other plant communities not only effectively reduces runoff and erosion [1,2,5,6,19] but also reduces SWS and increases SWC by vegetation and evapotranspiration [25] and change plant water relation in the man-made. Therefore, there should be limits to vegetation restoration in a region where the natural resources are scarce. The limit would depend on the capacity of the available natural resources in an ecosystem to support vegetation, i.e., land carrying capacity for vegetation or vegetation carrying capacity [14,18].

Water is the main factor influencing vegetation restoration and plant growth in most of the water-limited regions. Soil water resources are the most important resources. The carrying capacity of land for vegetation in this region is Soil Water Vegetation Carrying Capacity (SWVCC) [14,18] – the ability of soil water resources to support vegetation. It can be divided into space vegetation carrying capacity, soil water vegetation carrying capacity and soil fertility vegetation carrying capacity [13]. Therefore, the balance between the consumption and supply of water to the soil should be considered when restoring vegetation cover after some months or a couple of years in order to realize the goal of Sustainable Use of Soil Water Resources (SUSWR) and high-quality sustainable development of forest including soil and water conservation.

The Chinese Loess Plateau is the most serious soil and water losses in the world. It is located in the center part of China, and has an area of 642,000 km² of which 454,000 km² experiences soil and water loss and has scarce water resources. Soil in this region is very deep in the range of 30–80m from the surface [39], and the groundwater table is also deep [32]. Without irrigation, the best measure to solve issues of soil degradation and vegetation decline is to regulate the RBPGSW by reducing the population quantity of indicator plants in a plant community to match SWVCC on the Loess Plateau, thus balancing the soil water recharge and SWC is most important in plantations [14,17,18].

Plant water relationship is the most important relation in the water-limited regions. Drought is a recurring natural phenomenon. The complex nature and widespread impact of drought on forest and grass land with high coverage and production – driven by artificial vegetation consuming more than a permissible quantity of soil water resources in water-limited regions – means that regulating the RBPGSW is needed to maintain SWC of restoring vegetation at levels that sustainably use the soil water resources because man-made forest often use alien species and change the plant water relation, which easily lead to soil degradation and vegetation in dry years or waste in wet years. However, there is a lack of a universally accepted theory or method for regulating the RBPGSW and realizing Sustainable Use of Soil Water Resources (SUSWR) in forest and grass land in these regions. In this study, we aimed to develop the theory for regulating the RBPGSW in water-limited regions, including (1) the SWRULP and SWVCC and the sustainable utilization of soil and water resources.

Soil Water Resources

Soil is a loose medium with many apertures and a complex system. Soil can hold much water. Soil water is the water existing in the soil space, which is divided into capillary and non- capillary pores that change with soil depth and a state variable controlling a wide array of ecological, hydrological, geotechnical and meteorological processes by means of soil evaporation and plant transpiration. Soil water forms include crystalline water, solid water, vaporous water, tight or loose bound water, free water, gravitational water and anastatic water. Soil water cannot be transferred by humans from one place to another but can be used by plants. The amount of water held in the soil is the soil water resource. Soil water resources come from the concept of overall soil moistening proposed by M.I. Lvovich [11,14] and are water stored in the soil. Soil water resources can be defined for the needs of different disciplines such as forestry, agriculture, pedology and hydrology. For pedology, hydrology and Architecture, it is the water stored in the soil from surface soil to the groundwater table- generalized soil water resources. For agriculture, forestry and ranching, g, it is the water stored in the root zone soil- Soil water resources in narrow sense, and the dynamic soil water resources - the antecedent soil storage in the root zone plus the SWS from precipitation in the plant growing period or a year for evergreen plants – because soil water from precipitation in the growing season can be taken up immediately by live plants and influence their growth after rain event.

Soil water resources, the most important component of water resources and renewable water resources, can be divided into two parts: plant-unavailable water resources and plant- available water resources because some soil water resources can be used by plants, which is plant-available water, but the remainder is plant-unavailable water. Soil water adsorbed by solid soil different sizes of soil particles or held in the soil space. Soil water potential increases with increasing water molecule layers and the outside water molecules have higher water potential than the inside water molecule layer. Plant evapotranspiration including plant transpiration and film water evaporation on the surfaces of leaves, branches and stems when raining and after a rain event is a hydrological effect that induces soil suction. With water absorption by roots and evapotranspiration, soil water content and soil water potential are reduced but soil water suction is increased. When soil water content in every soil layer is reduced to designated soil moisture content, such as the wilting coefficient, the suction of soil particles to water exceeds that of plants to soil water and the water stored in all soil layers, soil water cannot be further taken up by plants in the MID. When soil water content in every soil layer is higher than wilting coefficient, the suction of soil particle to water is lower than that of plants to soil water, even the water plants sucked from the root soil does not meet the need of flowering and fruiting of plants for economy crop. The soil water resources in root zone when soil moisture content more than wilting coefficient are plant- available soil water resources.

SWRULP

Plant water relationship is the most important relation in the forest restoration of water-limited regions. Soil drying often happens man-made vegetation such as plantation, orchard, fuit or crop in water-limited regions. A tree or plant is a complex organism with a series of regulatory mechanisms to keep vital systems operating within appropriate restrictions, and with mechanisms to repair damage that may occur when these limits are exceeded. A tree or plant transports water from the soil to the atmosphere along a water potential gradient; its survival depends on maintenance of this transport system, which is important for maintaining hydration and efficient exchange of water for the carbon dioxide required for photosynthesis in a dynamic and often water-limited environment [1]. Plants also have some self- regulation function in the opening degree of stomata in leaves and the number of leaves retained during water stress; however, this self-adjustment is limited and cannot meet the need of regulating RBPGSW in the process of plant growth in some extreme conditions, such as severe drought and hot days on the Loess Plateau. While a plant grows, individual size expands, Canopies effect on SWS and SWC strengthens and roots deepen but soil water resources in plantation forest and grass land often decline in water-limited regions, even if there are some increases after rain events, and then root water uptake declines. Water stress begins when transpiration demand exceeds root water uptake, resulting in a loss of turgor. Subsequent short- and long- g- term responses include declines in cell enlargement and leaf expansion rate, reduced photosynthesis and transpiration. When soil water resources reduce to the degree, that results in severe soil drought, and finally leads to soil degradation, plant growth ceases and vegetation recedes or even dies.

It is important to regulate the RBPGSW for the prevention of soil drying and soil degradation, and the SUSWR in water-limited regions. In practice, regulating the RBPGSW is not required once soil drought happens in forest and grass land. This is because soil water mainly come from precipitation and there is a dramatical monthly and yearly change of precipitation in the water- limited region such as in the semiarid loess hilly region. According to weather data for 1983–2002 collected in Shanghuang Ecological Experimental Station and our study data for 2002–2016, Annual precipitation ranged from 284.3 mm in 1986 to 634.7 mm in 1984, see figure 1, and soil drying is a natural phenomenon and often occurs, and varying degrees of soil drying have different effects on plant growth especially at different growth stages, and there may be some water, which can buffer some effect of drought on plant growth when soil desiccation occurs. In addition, in the Loess Plateau, labour power is lacking and the level of mechanization is low. w. Machinery available for regulating the RBPGSM is inadequate for the huge areas of forest and grass land, often with rough terrain unsuited for much machinery.