1.2.5 Models of plant available wat

1.2.5 Models of plant available water
1.2.5.1 Historical models
The above-mentioned factors all influence the amount of water that a plant can extract from the soil. Until relatively recently, however, most of these factors have not been taken into account in models to calculate PAW. Early use of the term ‘PAW’ referred to the quantity of water in the soil ranging from a nominal ‘field capacity’ to a nominal ‘permanent wilting point’ (Veihmeyer and Hendrickson 1927). Field capacity referred to the soil water content at which excess water after a saturating rain or irrigation drained by gravitational force (in the absence of evaporation) over a period of several days. The concept was agronomic and was only loosely linked to a soil matric head (Simmonds et al. 1995). In fact, several different matric heads have been proposed in different parts of the world (Groenevelt et al. 2001). The ‘permanent wilting point’, PWP, referred to the soil water content remaining in the soil after plants wilted during the day and did not recover even if placed in an atmosphere of 100% relative humidity (Veihmeyer and Hendrickson 1949).
The range of soil water content between field capacity and permanent wilting point is rather imprecise and so cannot accurately define the amount of water that plants can extract from the soil. This concept of PAW relies on an assumption of equally available water between two critical potentials, field capacity and permanent wilting point. In reality, it is clear that the energy plants require to extract a unit of water from soil at field capacity is much lower than at the permanent wilting point. Furthermore, PAW is also influenced by other factors such as soil aeration, soil strength, hydraulic conductivity, and salinity. One might observe in some soils, for example that during drying, plants can extract soil water well beyond PWP. In the same soil when physical conditions are poor, however, water extraction may stop even when the soil is quite wet, and certainly before it reaches PWP. Richards and Wadleigh (1952) remarked that the concept plant water availability should involve two notions: the ability of plant roots to absorb and use water with which it is in contact, and the readiness or velocity with which the soil water moves into the root zone to replace that which has been taken up by the plant. Hillel (1971) felt less confident that soil properties alone could be used to predict soil water availability – he felt that soil water availability could only be assessed accurately using real plants in real soils under real meteorological conditions.
To integrate soil physical properties associated with plant growth into the concept of PAW, Letey (1985) introduced the qualitative concept called the Non-Limiting Water Range, NLWR, which referred to the range of soil water contents across which limitations to plant growth were negligible. Water uptake by roots was considered to be directly affected by soil physical conditions, particularly aeration and mechanical resistance. Accordingly the NLWR became narrower under conditions of poor aeration and high soil strength (Fig 1.4b) relative to that of soil of good structural condition (Fig 1.4a).
Da Silva et al. (1994) refined this concept to make it quantitative and called it the Least Limiting Water Range, LLWR. The LLWR merged the classical water contents at FC and PWP with those at critical limits of soil aeration and mechanical resistance. In wet soils the upper limit of LLWR corresponded to the water content at FC (matric head = 1 m), but if the volumetric air content was less than a cut-off value of 0.10 m3/m3 (selected as being important from historical literature), then the upper limit of LLWR was adjusted downward until the volumetric air content reached 0.10. At the dry end, water availability was thought to diminish due to increasing soil strength such that plant roots could not explore the soil to extract the water; many plant roots cannot grow into soils that have a penetration resistance > 2 MPa (Cockroft et al. 1969). If soil resistance to penetration was sufficiently low for root proliferation (< 2 MPa) then the lower limit of LLWR corresponded to the water content at 150 m. However, if penetration resistance was > 2 MPa before the soil dried to 150 m the lower limit of LLWR was adjusted upward to a water content where the soil resistance = 2 MPa. The effect of increasing bulk density on LLWR is shown in Figure 1.5.
Although LLWR incorporated some of the physical limitations affecting water availability (e.g. aeration, strength) it did not deal with other equally important limitations such as declining hydraulic conductivity and increasing osmotic stress in unsaturated soils. Furthermore, it used abrupt cut-off points, whereas real plants experience (and respond to) physical and chemical limitations in a gradual fashion rather than abruptly.
In earlier work, Feddes et al. (1978) introduced a model that used ‘reduction coefficients’ (varying between 0 and 1) to incorporate various physical and chemical limitations affecting soil water availability. Their ‘reduction functions’ were fairly simple (linear) and did not account for the complex nature of plant responses to soil limitations.
Eliminating the complexity of soil limiting factors (e.g. soil strength, aeration, hydraulic conductivity) that affect plant ability to extract water from soil, Minasny and McBratney (2003) introduced the concept of integral energy which focuses on the quantity of energy required by the plant to remove a unit amount of water from the soil. The energy was calculated from the integral of the soil water retention curve. This work indicated that the energy required to remove the same amount water from a silty clay soil (within the range FC and PWP) was almost 1.5 times higher than that of clay soil (Minasny and McBratney 2003). The integral energy concept is important in terms of plant physiology as it considers the plant energy requirements for water uptake.

Kết quả (Việt) 1:

1.2.5 mô hình của thực vật có nước1.2.5.1 lịch sử mô hìnhNhững yếu tố nêu trên tất cả ảnh hưởng đến lượng nước mà một nhà máy có thể trích xuất từ đất. Cho đến khi tương đối gần đây, Tuy nhiên, hầu hết các yếu tố này đã không được đưa vào tài khoản trong các mô hình tính toán PAW. Sớm sử dụng thuật ngữ 'Chân' gọi số lượng nước trong đất khác nhau, từ một danh nghĩa 'lĩnh vực khả năng' để một danh nghĩa 'vĩnh viễn héo điểm' (Veihmeyer và Hendrickson 1927). Lĩnh vực khả năng gọi hàm lượng nước đất lúc đó nước dư thừa sau khi một mưa saturating hoặc thủy lợi đầu nguồn của lực hấp dẫn (trong sự vắng mặt của bay hơi) trong một khoảng thời gian vài ngày. Khái niệm là nông học và chỉ một cách lỏng lẻo được liên kết với một cái đầu đất matric (Simmonds et al. 1995). Trong thực tế, một số người đứng đầu matric khác nhau đã được đề xuất ở các bộ phận khác nhau của thế giới (Groenevelt et al. năm 2001). Các 'vĩnh viễn héo điểm', PWP, gọi hàm lượng nước đất còn lại trong đất sau khi nhà máy wilted trong ngày và đã không phục hồi ngay cả khi đặt trong một bầu không khí của 100% độ ẩm tương đối (Veihmeyer và Hendrickson 1949).The range of soil water content between field capacity and permanent wilting point is rather imprecise and so cannot accurately define the amount of water that plants can extract from the soil. This concept of PAW relies on an assumption of equally available water between two critical potentials, field capacity and permanent wilting point. In reality, it is clear that the energy plants require to extract a unit of water from soil at field capacity is much lower than at the permanent wilting point. Furthermore, PAW is also influenced by other factors such as soil aeration, soil strength, hydraulic conductivity, and salinity. One might observe in some soils, for example that during drying, plants can extract soil water well beyond PWP. In the same soil when physical conditions are poor, however, water extraction may stop even when the soil is quite wet, and certainly before it reaches PWP. Richards and Wadleigh (1952) remarked that the concept plant water availability should involve two notions: the ability of plant roots to absorb and use water with which it is in contact, and the readiness or velocity with which the soil water moves into the root zone to replace that which has been taken up by the plant. Hillel (1971) felt less confident that soil properties alone could be used to predict soil water availability – he felt that soil water availability could only be assessed accurately using real plants in real soils under real meteorological conditions. To integrate soil physical properties associated with plant growth into the concept of PAW, Letey (1985) introduced the qualitative concept called the Non-Limiting Water Range, NLWR, which referred to the range of soil water contents across which limitations to plant growth were negligible. Water uptake by roots was considered to be directly affected by soil physical conditions, particularly aeration and mechanical resistance. Accordingly the NLWR became narrower under conditions of poor aeration and high soil strength (Fig 1.4b) relative to that of soil of good structural condition (Fig 1.4a).Da Silva et al. (1994) refined this concept to make it quantitative and called it the Least Limiting Water Range, LLWR. The LLWR merged the classical water contents at FC and PWP with those at critical limits of soil aeration and mechanical resistance. In wet soils the upper limit of LLWR corresponded to the water content at FC (matric head = 1 m), but if the volumetric air content was less than a cut-off value of 0.10 m3/m3 (selected as being important from historical literature), then the upper limit of LLWR was adjusted downward until the volumetric air content reached 0.10. At the dry end, water availability was thought to diminish due to increasing soil strength such that plant roots could not explore the soil to extract the water; many plant roots cannot grow into soils that have a penetration resistance > 2 MPa (Cockroft et al. 1969). If soil resistance to penetration was sufficiently low for root proliferation (< 2 MPa) then the lower limit of LLWR corresponded to the water content at 150 m. However, if penetration resistance was > 2 MPa before the soil dried to 150 m the lower limit of LLWR was adjusted upward to a water content where the soil resistance = 2 MPa. The effect of increasing bulk density on LLWR is shown in Figure 1.5.Although LLWR incorporated some of the physical limitations affecting water availability (e.g. aeration, strength) it did not deal with other equally important limitations such as declining hydraulic conductivity and increasing osmotic stress in unsaturated soils. Furthermore, it used abrupt cut-off points, whereas real plants experience (and respond to) physical and chemical limitations in a gradual fashion rather than abruptly.In earlier work, Feddes et al. (1978) introduced a model that used ‘reduction coefficients’ (varying between 0 and 1) to incorporate various physical and chemical limitations affecting soil water availability. Their ‘reduction functions’ were fairly simple (linear) and did not account for the complex nature of plant responses to soil limitations.Eliminating the complexity of soil limiting factors (e.g. soil strength, aeration, hydraulic conductivity) that affect plant ability to extract water from soil, Minasny and McBratney (2003) introduced the concept of integral energy which focuses on the quantity of energy required by the plant to remove a unit amount of water from the soil. The energy was calculated from the integral of the soil water retention curve. This work indicated that the energy required to remove the same amount water from a silty clay soil (within the range FC and PWP) was almost 1.5 times higher than that of clay soil (Minasny and McBratney 2003). The integral energy concept is important in terms of plant physiology as it considers the plant energy requirements for water uptake.