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Advances in Nitrogen Loss Leached by Precipitation from Plant Canopy
Available online at www.sciencedirect.com
Agricultural Sciences in China
2008, 7(4): 480-486
ScienceDirect
April 2008
Advances in Nitrogen Loss Leached by Precipitation from Plant Canopy LI Shi-qingl,Jl Chun-rongl.2,FANG Ya-ningl.2,CHEN Xiao-lil and LI Sheng-xiu2 State Key Laboratory of Soil Erosion and Dryland Farming on Loess Plateadlnstitute of Soil and Water Conservation, Chinese Academy of Sciences/Ministry of Water Resources, Yangling 712100, P.R.China 2 Department of Natural Resources and Environment, Northwest A & F University, Yangling 712100, P.R.China I
Abstract Function of canopy in changing nutrient cycle and flux is one of the focuses in recent years. On the basis of comprehensively appraising published research, we analyzed the nitrogen loss leaching from plant canopy and several factors which affected it. We pointed out the disadvantages of the published researches and the key issues that ought to be solved: (1) The menstruation need to be advanced, and the research should be carried out on nitrogen loss leaching from the canopy of the field plant. (2) If the nitrogen is leached from the plant canopy, the research on the type of nitrogen loss should be carried out, and the nitrogen use efficiency of different varieties should be dealt on a research perspective with regard to the nitrogen leaching. (3) The research should be conducted on the mechanism and pathway, and the progress of nitrogen leaching; and the factors affecting nitrogen leaching should be included in the research, such as the leaf area of different growth stages, stomata densities, stomata conductance, and the apparent free space, which are beneficial to explain the mechanism of nitrogen leaching from the plant canopy.
Key words: plant canopy, nitrogen loss, research advances
INTRODUCTION Plant canopy plays an important role in nutrient circulation and changing of flux in nutrient circulation, and it is a common scientific issue in plant nutrition and ecology (Prescott 2002). Nitrogen losses in plant canopy and nitrogen absorption of plants are an important component of soil-plant nutrient circulation system, which have a same important role in nitrogen utilization and ecosystem nitrogen equilibrium. Studies showed that crop canopy nitrogen losses in plant middlelater growth stage accounted for 20-30% of canopy maximum N accumulation, with an average of 25%, and there was a 5% of them loss as NH,, and there has been no idea of the rest of 20% (Li et al. 1995). Therefore, investigation of the flux and the destination
of N loss from the canopy has a great scientific value on further understanding of plant N mechanism, equilibrium, and improving N utilization. Most of the previous studies focus on N absorption and utilization with no further investigation on N losses (Schjoerring et al. 2000). This article focuses on reviewing the recent studies on plant canopy N losses and its influence factors in order to deepen understanding on farmland ecosystem N equilibrium more accurately and reliably.
PLANT CANOPY NITROGEN LOSS Some previous researches found that the N accumulation above ground reached a maximum in the most vigorous period of the plant growth, and decreased dramatically thereafter; plant canopy N decreased obviously in the
This paper is translated from its Chinese version in Scientia Agricultura Sinica. LI Shi-qing, Tel: +86-29-87016171,E-mail: [email protected]
02008, CAAS. All rightsreserved. Pul lished by Elsevter Ltd.
Advances in Nitrogen Loss Leached by Precipitation from Plant Canopy
mid-later period of the plant growth, and 1/3-1/5 of the maximum N volume were lost during that period, but the lost N were not found in the root system (Li and Li 2001). Mattson et al. (1998) indicated that 75 kg N ha-' NH, were volatilized in 10 weeks in some plants and volatilized NH, of winter wheat was 8.1 kg N ha-' before flowering and 7.4 kg N ha-' after flowering (Markewotz et al. 2004). Schjoerring and Mattsson (2001) used rape, wheat, barley, and pea as experimental materials, and the results showed that the volatized NH, from plant canopy ranged 1-5 kg N ha-', which accounted for 14% of the total fertilized N. Plant canopy nutrient loss is one of the research issues dealt upon both nationally and internationally; it is also the research foreland and intercross of plant ecology, plant physiology, and plant nutrition (Prescott 2002). However, there is relatively little research on it in China compared to other countries. In countries other than China, the research objects are mainly forest canopy electropositive ions such as the leaching of K+, Ca2+,Mg2+,Na+ and so on. 15Nfootprint experiment proved that to half wet wheat-corn rotate system which need a lot of N, the N using efficiency was 60%,leaching loss was 8%, volatilization loss was 15%, 7% becomes soil organic N, 3% was lost by canopy volatilization, but there was still 7% N whose destination was unknown (Li and Li 2001). According to the research about forest canopy nutrient leaching losses, it could be presumed that this part of N is lost by canopy leaching losses. The previous researches about nutrient leaching losses mainly focused on the influence of precipitation to soil nutrient leaching losses and the influence on plant growth after the leaching losses, especially the soil NO,-N losses, but lack of researches on the canopy nutrient leaching.
APPROACH OF NUTRIENT LOSS More and more researches (Li et al. 2004; Hill et al. 2001) showed that plant canopy nutrient loss had two possible ways: One was nutrient loss in term of gas during senescence; the other one was by nutrient leaching. Schjoerring et al. (2002) further studied the mechanism, condition, and the influence factors of nutrient loss in the gaseous form. But the study on the canopy nutrient leaching loss was lacked. Nutrient loss
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of canopy is regarding the matter of the above-ground of plants removed by the function of water such as rain, irrigation water, dew and fog, which includes the matter excreted initially or by guttation, lost from the injured place of a plant and leached from the apoplastic of leaf (Mccammon 1980). Lots of materials inside plants including inorganic and organic matter could be leached out from the above ground of plants, and they are K, Ca, Mg, and Mn (Sayer and Fahey 1999). G r a m and Mass (1988) showed that the plants growing in the field had a difference of the concentration of leaf K, Ca, Mg, and Na in that of in the greenhouse, but no significiant difference in Zn and P. The reason is that K, Ca, Mg, and Mn can be leached by raining. If plants were grown outside and at the same time covered from rain, there was no difference between the field and greenhouse plants. Plant apoplastic consists of the fiber of cell wall, mini-crystal, and cell clearance. Leaching can take out inorganic and organic matter from apoplastic of leaves, so it has important ecological function and is considered by the researchers. The ion concentration is significantly different in different leaf apoplastic, for example, K+can be 50 to100 mmol. Besides the mineral nutrients stored by the plant itself, the leaves can excrete some harmful mineral, or excrete the over absorbed mineral nutrient by the root system to apoplastic. Apoplastic has limited ability to store nutrient, so the ions have 5 possible approaches: (1) leaching by crystalline, (2) ejecting out by guttation, (3) leaching, (4) storing in epidermis, and ( 5 ) leaving the plant by senescence (Li and Zhang 1997). As the volume of leaf apoplastic is very small, and therefore, little quantity of ions can increase the water potential significantly. Since the plant cell is a permeable system, the platform of metabolizingis cell membrane, the substance entering cell from outside environment must go through and function with membrane, therefore, once membrane is influenced, the penetration will become bigger so that electrolyte percolate outside, and the nutrient is lost.
CURRENT RESEARCH METHODS OF PLANT CANOPY NUTRIENT LOSS The research about plant canopy nutrient loss is limited
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to forest canopy, and calculated using subtraction method (Lin et al. 2001; Staelens et al. 2003): Quantifying the nutrient loss by measuring the difference of nutrient concentration of throughfall, stemflow, and bulk precipitation, known as “miniumbrellamethod”. Besides “miniumbrellamethod”, some researchers quantified the nutrient loss by “ion exchange process” (Fenn and Poth 1996). Both “miniumbrellamethod” and “ion exchange process” can not differentiate the precipitation nutrient loss through the plant canopy, which comes from the plant itself or from dry-wet deposition especially the soil and dust on the leaves. Previous researchers did not study the mechanism of nutrient leaching loss, therefore leaching is difficult to be explained in a theoretical level (Figs.1 and 2).
MECHANISM OF PLANT CANOPY NUTRIENT LOSS Recently, the study of the plant-water relations has become one of the most important issues of plant ecophysiology (Li et al. 2000). The study found that the influence of precipitation leaching to plant growth was that leaching can lead to nutrient loss in certain conditions, thereby directly affected the plant physiological metabolism process (Tang et al. 2004). Leaching can take out inorganic and organic matter from complete leaves so that it has meaningful ecological function. The concentration of mineral nutrient in plants after precipitation leaching is decided by two factors: First, the loss by leaching, and second, it is the supplementary amount through roots assimilating to xylem. The leaching quantity is decided by the leachable ion quantity and the characteristics of leaf protective tissues. Meanwhile because of the differences of gene types, known as the differences on salt resistant mechanisms, mineral nutrient assimilation and transportation, and the leaf leaching resistant ability, the completely different leaching results could be obtained (Herrmann et aE. 2005). The large quantity of mineral N and small molecule organic N in apoplastic which count for 520% of living organism including cell clearance is considered the main source of leaching nutrient especially the solvable nutrient (Maher et al.
2005). Grogan and Chapin (2000) found that in evergreen forests and deciduous forests, the N, P, K from canopy leaching made up for 14-15%, 15%, and 48-59% of total canopy nutrient. Usually, the leached nutrient mainly comes from leaves; the rate can be as high as above 55%, and leaching happens in the growing period in which the nutrient concentration in cell gap and the solvency are high, especially in the fast growing period of leaf and the senescence period. The experiment (Xia et at. 2000) also showed that leaching mainly influences the ion balance of apoplastic; K+,and Na+ are easier to be leached than Ca2+and Mg2+, because univalent ions easily penetrate cutin, moreover, univalent ions have smaller bondage than divalent ions in the positive ion exchange area between cutin and cell wall. Generally in plants, there is no significant difference between leaching and without leaching for the root mineral nutrient, and this is because in the normal nutrient supply condition, the leaching loss above ground can be compensated by the roots faster assimilation. But when the nutrients are limited, the nutrient concentration of plants decreased significantly after leaching (Fitamorris et al. 2006). The research objects of plant canopy nutrient leaching are mainly forest canopy electropositive ions such as the leaching of K+, Ca2+,Mg2+,Na’ and so on. Tian (2002) indicated that the concentration of P, K, Ca, Mg, Fe, Cu, Mn, Zn, and Si was higher in fir wood forest than in the rain, and the nutrient leached from fir forest canopy was 79.475 kg ha-’yr-l. The total nutrient from canopy through forest rain to the forest soil is 161.957 kg ha-’ yrl. In fir forest in Guangping, China, the inorganic N concentration in the rain is 0.453 mg kg-’, but the inorganic N concentration in throughfall water and stemflow water is 3 to 5 times higher than that in the rain. The organic N concentration changes is not significant, and the concentration of P, K, Ca, Mg, Fe, Cu, Mn, Zn, and Si are all increased, especially, Mg and Mn. The leached minerals are beneficial to the soil nutrient supply.
INFLUENCE FACTORS OF PLANT CANOPY NUTRIENT LOSS The influence of precipitation on nutrient leaching loss
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Advances in Nitrogen Loss Leached by Precipitation from Plant Canopy
depends on precipitation ingredient, environmental conditions, the leaching resistant characteristics of plants (leaf surface protective tissues), and plant adaptation mechanism under stress (Cunie et al. 1999). The previous study on forest canopy showed that, the influence and mechanism of precipitation on forest canopy nutrient loss such as precipitation chemistry, plant nutrient ion exchange, and assimilation (Berger and Glatzel 1998), the electropositive ion leaching quantity of plant canopy, depends on not only the local climate, geographical conditions, and forest ecosystem conditions, but also the structure and physiological conditions of forest canopy (Draaijers
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and Erisman 1995). When the quantity of precipitation is larger, the nutrients stayed for a lesser time in the upper part of the canopy and mainly focused on leaching the material on the leaf surface (Draaijers et al. 1988). Because of the influence of canopy blocking, with the continual droping and penetrating of precipitation, the water flow decreases and has more time touching the leaves, and the H’ in the precipitation will exchange with the electropositive ion in the leaf tissue. In the beginning of the precipitation, its function is mainly leaching the dropped materials on the leaf surface, and there is a buffer process of forest canopy for precipitation acidity because the exchange
Fig. 1 Instruments for collecting SF and BP with “miniumbrella method”
Fig. 2 Instruments for collecting TF with “miniumbrella method”
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reaction of electropositive ion needs a completely wet leaf surface and the process is influenced by the quantity and acidity of precipitation, but the NH,+ in ions has certain competitive characteristics whicls. will be shown in the leaching quantity when H+reacts with electropositive ion in the leaves (Vanek and Draaijers 1994). The nutrient type, nutrient concentration (especially the concentrationof the dissolvable nutrient), contacting time, plant type, and precipitation acidity influence the velocity of nutrient leaching and quantity (Currie 1999). It is generally agreed that in the very beginning of the precipitation, the leaching velocity reaches the maximum, after that it decreases exponentially (Mccammon 1980), so the frequency of precipitation is more important than the intensity of precipitation for the canopy nutrient leaching. But there is no enough evidence to explain whether the dirt on the leaves leads to the exaggerating of the measurement in the beginning of the precipitation. The leaching quantity is different when the nutrient type is different. It is generally considered that the leached N and P are the same. Under the annual precipitation of 3 325 mm, Xu and Hirata (2005) investigated the nutrient leaching in sub-tropical evergreen forest canopy in Okinawa Island in Japan by “miniumbrellamethod” and found that the N and P from the canopy entering to the soil are 43 and 2.6 kg annually, respectively. Staelens et al. (2003) studied the leaching of the electropositive ion from throughflow and stem flow of two deciduous oak forest canopies and found that the leaching quantity of electropositive ion especially Ca2+ and Mg2+had something to do with the study area. The leaching quantity of Ca2+and Mg2+in Chile are 22.5 and 12.2 kg ha-’ yrl and in Flanders are 12.9 and 3.7 kg ha-’ yr’. The reason for leading to the different leaching quantity in the two areas is that the quantity of electropositiveions in soil and leaves is different in those two areas; the quantity of electropositive ions in soil and leaves is significantlyhigher in Chile thanin Flanders. Duchesne et al. (2001) found that in northern Canada there was a lack of N in soil but plenty of N in precipitation, and there was leaching of electropositive ions such as Ca2+,K+,Mg2+and P; forest canopy could assimilate all the N from precipitation, where the assimilation quantity made up to 65% of the maximum
LI Shi-qing et al.
leaf N storage. He thought that it was the absolute result of the severe shortage of N in the canopy. However, more studies had opposite results: Gallaway and Nadkarni (1991) through “miniumbrella method” found that when compared with the bare land precipitation, the N in TF through canopy increased by 45-60% and the N leaching intensity had significant seasonal changes. Swank and Reynolds (1987) found that compared with BP, NO,--N significantly increased in TF, NO,--N from leaf leaching made up to 82.8%of the total quantity from canopy leaching, and it indicated that NO,--N was the main inorganic N from canopy leaching. Besides plant itself (such as plant reproductiveperiod, nutrient condition, leaf protective tissue) and environmental conditions(temperature, illumination), the canopy nutrient leaching quantity is mainly related to the pH of the precipitation because leaf apoplastic involves the adjustment of cell pH, and the ion moving ability is very different in different pH so that different pH of the precipitation will have significant influence on the leaching result (Mccammon 1980; Sayer and Fahey 1999). Xia et al. (2002) studied the influence of different leaching liquid with different pH, and NaCl stress on the N constitute of the corn canopy leaching materials, and it showed that the quantity of NO,--N, NH,+-N, Org-N, and Tot-N from leaching materials increase with the increase of the plant biomass. The quantity of NH,+-N, NO,--N and Tot-N are higher on leaching with pH 3.5 than pH 7.0, but lower in Org-N quantity. When leaching is carried out using pH 7.0, the N in leaching liquid is mainly Org-N, which makes up 48.3-50.5%,but when using pH 3.5, the N is mainly NO,--N. Leaching at pH 3.5 can increase NO,--N and Tot-N quantity in terms of unit wet weight. N is an essential mineral nutrient in plants, and each type of N quantity from leaching material increases with the increase of the plant growth. It is mainly attributed to the plant growth such as the increase of leaves number and canopy structure, the increase of leachable area, and the accumulation of nutritious matter, as well as, the increase of ion concentration of apoplastic which was caused by the increase of penetration ability of cell membrane, wind injury, and so on. All the factors significantly increase the ion concentration of leaching materials.
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Advances in Nitrogen Loss Leached by Precipitation from Plant Canopy
PROBLEMS AND PROSPECT A progress has been achieved on plant canopy nutrient leaching loss. For example, the nutrient loss during precipitation in forest canopy has been demonstrated; NO,--N has been found to be an important inorganic N type, and so on. But there are still some questions existed to be clarified: (1) The method should be improved. “miniumbrella method” and “ion exchange process” can not precisely differentiatethe canopy nutrient loss which comes from the plant itself, or from the soil and dirt on the canopy surface, so the more precise and reliable methods are needed. (2) Previous researches focused on the study of the forest canopy, but few studies on the farmland plant canopy nutrient leaching loss. Because of fertilization, there is more N, especially mineral N and small molecules of organic N in crop canopy free spaces than in forest canopies. It is indicated that the leaching N source is bigger, but whether the canopy N leaching exists or not needs to be studied. And if the canopy N leaching does exist, it should be studied when researching on N flow and canopy N leaching loss in farmland ecosystem, however, the current research on N flow and canopy N leaching loss in farmland ecosystem does not include this part. (3) If there is a canopy N leaching loss in farmland crops, the factors which influence canopy N leaching loss, such as whether soil N supplement, precipitation period, and precipitationintensityinfluencecanopy N leaching should be further studied. (4)N leaching type should be further studied. The previous researches on N leaching type mainly focused on NO,--N and NH,+-N, and NO,--N was found to be the important inorganic N type, but it is seldom reported whether there is an organic N type, especially, the small molecule solvable N. (5) It is required to research canopy N leaching from plants with different N efficient gene types together. There are fewer reports to indicate whether there is a difference of canopy N leaching for different N efficiency gene types, and whether the canopy leaching is the reason for leading to the decrease of the use efficiency of plant N. (6) No research has existed on discovering the mechanism of the approach and process of canopy N loss, so the theoretical support is lacked on plant canopy N leaching loss. Based on the current research on forest canopy nutrient leaching, free spaces and stomata are
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possibly the main channels of nutrient leaching, and they change with the growth period. Therefore, combining the researches on the leaf area in different growth periods, stomata densities, stomata conductivity, leaf free spaces, and solvable N in the free space, and the research on canopy leaching, is beneficial to further discover the mechanism of canopy N leaching loss and provide theoretical support on canopy N leaching loss. However, there are not so many researches about that currently. Therefore, there is an urgent need to carry out the relevant researches on farmland crop canopy N leaching, find out other approaches of farmland crop canopy N loss except the loss by evaporating in terms of gas, and provide a scientific basis on further understanding plant canopy N loss.
Acknowledgements This work was supported by the National Natural Science Foundation of China (30670326, 3057 1116), and the Talent Program in West China of Chinese Academy of Sciences (2006LH01).
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Q2008. CAAS. All rights reserved. Publishedby Elsevierltd.
Agricultural Sciences in China
2008, 7(4): 480-486
ScienceDirect
April 2008
Advances in Nitrogen Loss Leached by Precipitation from Plant Canopy LI Shi-qingl,Jl Chun-rongl.2,FANG Ya-ningl.2,CHEN Xiao-lil and LI Sheng-xiu2 State Key Laboratory of Soil Erosion and Dryland Farming on Loess Plateadlnstitute of Soil and Water Conservation, Chinese Academy of Sciences/Ministry of Water Resources, Yangling 712100, P.R.China 2 Department of Natural Resources and Environment, Northwest A & F University, Yangling 712100, P.R.China I
Abstract Function of canopy in changing nutrient cycle and flux is one of the focuses in recent years. On the basis of comprehensively appraising published research, we analyzed the nitrogen loss leaching from plant canopy and several factors which affected it. We pointed out the disadvantages of the published researches and the key issues that ought to be solved: (1) The menstruation need to be advanced, and the research should be carried out on nitrogen loss leaching from the canopy of the field plant. (2) If the nitrogen is leached from the plant canopy, the research on the type of nitrogen loss should be carried out, and the nitrogen use efficiency of different varieties should be dealt on a research perspective with regard to the nitrogen leaching. (3) The research should be conducted on the mechanism and pathway, and the progress of nitrogen leaching; and the factors affecting nitrogen leaching should be included in the research, such as the leaf area of different growth stages, stomata densities, stomata conductance, and the apparent free space, which are beneficial to explain the mechanism of nitrogen leaching from the plant canopy.
Key words: plant canopy, nitrogen loss, research advances
INTRODUCTION Plant canopy plays an important role in nutrient circulation and changing of flux in nutrient circulation, and it is a common scientific issue in plant nutrition and ecology (Prescott 2002). Nitrogen losses in plant canopy and nitrogen absorption of plants are an important component of soil-plant nutrient circulation system, which have a same important role in nitrogen utilization and ecosystem nitrogen equilibrium. Studies showed that crop canopy nitrogen losses in plant middlelater growth stage accounted for 20-30% of canopy maximum N accumulation, with an average of 25%, and there was a 5% of them loss as NH,, and there has been no idea of the rest of 20% (Li et al. 1995). Therefore, investigation of the flux and the destination
of N loss from the canopy has a great scientific value on further understanding of plant N mechanism, equilibrium, and improving N utilization. Most of the previous studies focus on N absorption and utilization with no further investigation on N losses (Schjoerring et al. 2000). This article focuses on reviewing the recent studies on plant canopy N losses and its influence factors in order to deepen understanding on farmland ecosystem N equilibrium more accurately and reliably.
PLANT CANOPY NITROGEN LOSS Some previous researches found that the N accumulation above ground reached a maximum in the most vigorous period of the plant growth, and decreased dramatically thereafter; plant canopy N decreased obviously in the
This paper is translated from its Chinese version in Scientia Agricultura Sinica. LI Shi-qing, Tel: +86-29-87016171,E-mail: [email protected]
02008, CAAS. All rightsreserved. Pul lished by Elsevter Ltd.
Advances in Nitrogen Loss Leached by Precipitation from Plant Canopy
mid-later period of the plant growth, and 1/3-1/5 of the maximum N volume were lost during that period, but the lost N were not found in the root system (Li and Li 2001). Mattson et al. (1998) indicated that 75 kg N ha-' NH, were volatilized in 10 weeks in some plants and volatilized NH, of winter wheat was 8.1 kg N ha-' before flowering and 7.4 kg N ha-' after flowering (Markewotz et al. 2004). Schjoerring and Mattsson (2001) used rape, wheat, barley, and pea as experimental materials, and the results showed that the volatized NH, from plant canopy ranged 1-5 kg N ha-', which accounted for 14% of the total fertilized N. Plant canopy nutrient loss is one of the research issues dealt upon both nationally and internationally; it is also the research foreland and intercross of plant ecology, plant physiology, and plant nutrition (Prescott 2002). However, there is relatively little research on it in China compared to other countries. In countries other than China, the research objects are mainly forest canopy electropositive ions such as the leaching of K+, Ca2+,Mg2+,Na+ and so on. 15Nfootprint experiment proved that to half wet wheat-corn rotate system which need a lot of N, the N using efficiency was 60%,leaching loss was 8%, volatilization loss was 15%, 7% becomes soil organic N, 3% was lost by canopy volatilization, but there was still 7% N whose destination was unknown (Li and Li 2001). According to the research about forest canopy nutrient leaching losses, it could be presumed that this part of N is lost by canopy leaching losses. The previous researches about nutrient leaching losses mainly focused on the influence of precipitation to soil nutrient leaching losses and the influence on plant growth after the leaching losses, especially the soil NO,-N losses, but lack of researches on the canopy nutrient leaching.
APPROACH OF NUTRIENT LOSS More and more researches (Li et al. 2004; Hill et al. 2001) showed that plant canopy nutrient loss had two possible ways: One was nutrient loss in term of gas during senescence; the other one was by nutrient leaching. Schjoerring et al. (2002) further studied the mechanism, condition, and the influence factors of nutrient loss in the gaseous form. But the study on the canopy nutrient leaching loss was lacked. Nutrient loss
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of canopy is regarding the matter of the above-ground of plants removed by the function of water such as rain, irrigation water, dew and fog, which includes the matter excreted initially or by guttation, lost from the injured place of a plant and leached from the apoplastic of leaf (Mccammon 1980). Lots of materials inside plants including inorganic and organic matter could be leached out from the above ground of plants, and they are K, Ca, Mg, and Mn (Sayer and Fahey 1999). G r a m and Mass (1988) showed that the plants growing in the field had a difference of the concentration of leaf K, Ca, Mg, and Na in that of in the greenhouse, but no significiant difference in Zn and P. The reason is that K, Ca, Mg, and Mn can be leached by raining. If plants were grown outside and at the same time covered from rain, there was no difference between the field and greenhouse plants. Plant apoplastic consists of the fiber of cell wall, mini-crystal, and cell clearance. Leaching can take out inorganic and organic matter from apoplastic of leaves, so it has important ecological function and is considered by the researchers. The ion concentration is significantly different in different leaf apoplastic, for example, K+can be 50 to100 mmol. Besides the mineral nutrients stored by the plant itself, the leaves can excrete some harmful mineral, or excrete the over absorbed mineral nutrient by the root system to apoplastic. Apoplastic has limited ability to store nutrient, so the ions have 5 possible approaches: (1) leaching by crystalline, (2) ejecting out by guttation, (3) leaching, (4) storing in epidermis, and ( 5 ) leaving the plant by senescence (Li and Zhang 1997). As the volume of leaf apoplastic is very small, and therefore, little quantity of ions can increase the water potential significantly. Since the plant cell is a permeable system, the platform of metabolizingis cell membrane, the substance entering cell from outside environment must go through and function with membrane, therefore, once membrane is influenced, the penetration will become bigger so that electrolyte percolate outside, and the nutrient is lost.
CURRENT RESEARCH METHODS OF PLANT CANOPY NUTRIENT LOSS The research about plant canopy nutrient loss is limited
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to forest canopy, and calculated using subtraction method (Lin et al. 2001; Staelens et al. 2003): Quantifying the nutrient loss by measuring the difference of nutrient concentration of throughfall, stemflow, and bulk precipitation, known as “miniumbrellamethod”. Besides “miniumbrellamethod”, some researchers quantified the nutrient loss by “ion exchange process” (Fenn and Poth 1996). Both “miniumbrellamethod” and “ion exchange process” can not differentiate the precipitation nutrient loss through the plant canopy, which comes from the plant itself or from dry-wet deposition especially the soil and dust on the leaves. Previous researchers did not study the mechanism of nutrient leaching loss, therefore leaching is difficult to be explained in a theoretical level (Figs.1 and 2).
MECHANISM OF PLANT CANOPY NUTRIENT LOSS Recently, the study of the plant-water relations has become one of the most important issues of plant ecophysiology (Li et al. 2000). The study found that the influence of precipitation leaching to plant growth was that leaching can lead to nutrient loss in certain conditions, thereby directly affected the plant physiological metabolism process (Tang et al. 2004). Leaching can take out inorganic and organic matter from complete leaves so that it has meaningful ecological function. The concentration of mineral nutrient in plants after precipitation leaching is decided by two factors: First, the loss by leaching, and second, it is the supplementary amount through roots assimilating to xylem. The leaching quantity is decided by the leachable ion quantity and the characteristics of leaf protective tissues. Meanwhile because of the differences of gene types, known as the differences on salt resistant mechanisms, mineral nutrient assimilation and transportation, and the leaf leaching resistant ability, the completely different leaching results could be obtained (Herrmann et aE. 2005). The large quantity of mineral N and small molecule organic N in apoplastic which count for 520% of living organism including cell clearance is considered the main source of leaching nutrient especially the solvable nutrient (Maher et al.
2005). Grogan and Chapin (2000) found that in evergreen forests and deciduous forests, the N, P, K from canopy leaching made up for 14-15%, 15%, and 48-59% of total canopy nutrient. Usually, the leached nutrient mainly comes from leaves; the rate can be as high as above 55%, and leaching happens in the growing period in which the nutrient concentration in cell gap and the solvency are high, especially in the fast growing period of leaf and the senescence period. The experiment (Xia et at. 2000) also showed that leaching mainly influences the ion balance of apoplastic; K+,and Na+ are easier to be leached than Ca2+and Mg2+, because univalent ions easily penetrate cutin, moreover, univalent ions have smaller bondage than divalent ions in the positive ion exchange area between cutin and cell wall. Generally in plants, there is no significant difference between leaching and without leaching for the root mineral nutrient, and this is because in the normal nutrient supply condition, the leaching loss above ground can be compensated by the roots faster assimilation. But when the nutrients are limited, the nutrient concentration of plants decreased significantly after leaching (Fitamorris et al. 2006). The research objects of plant canopy nutrient leaching are mainly forest canopy electropositive ions such as the leaching of K+, Ca2+,Mg2+,Na’ and so on. Tian (2002) indicated that the concentration of P, K, Ca, Mg, Fe, Cu, Mn, Zn, and Si was higher in fir wood forest than in the rain, and the nutrient leached from fir forest canopy was 79.475 kg ha-’yr-l. The total nutrient from canopy through forest rain to the forest soil is 161.957 kg ha-’ yrl. In fir forest in Guangping, China, the inorganic N concentration in the rain is 0.453 mg kg-’, but the inorganic N concentration in throughfall water and stemflow water is 3 to 5 times higher than that in the rain. The organic N concentration changes is not significant, and the concentration of P, K, Ca, Mg, Fe, Cu, Mn, Zn, and Si are all increased, especially, Mg and Mn. The leached minerals are beneficial to the soil nutrient supply.
INFLUENCE FACTORS OF PLANT CANOPY NUTRIENT LOSS The influence of precipitation on nutrient leaching loss
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Advances in Nitrogen Loss Leached by Precipitation from Plant Canopy
depends on precipitation ingredient, environmental conditions, the leaching resistant characteristics of plants (leaf surface protective tissues), and plant adaptation mechanism under stress (Cunie et al. 1999). The previous study on forest canopy showed that, the influence and mechanism of precipitation on forest canopy nutrient loss such as precipitation chemistry, plant nutrient ion exchange, and assimilation (Berger and Glatzel 1998), the electropositive ion leaching quantity of plant canopy, depends on not only the local climate, geographical conditions, and forest ecosystem conditions, but also the structure and physiological conditions of forest canopy (Draaijers
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and Erisman 1995). When the quantity of precipitation is larger, the nutrients stayed for a lesser time in the upper part of the canopy and mainly focused on leaching the material on the leaf surface (Draaijers et al. 1988). Because of the influence of canopy blocking, with the continual droping and penetrating of precipitation, the water flow decreases and has more time touching the leaves, and the H’ in the precipitation will exchange with the electropositive ion in the leaf tissue. In the beginning of the precipitation, its function is mainly leaching the dropped materials on the leaf surface, and there is a buffer process of forest canopy for precipitation acidity because the exchange
Fig. 1 Instruments for collecting SF and BP with “miniumbrella method”
Fig. 2 Instruments for collecting TF with “miniumbrella method”
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reaction of electropositive ion needs a completely wet leaf surface and the process is influenced by the quantity and acidity of precipitation, but the NH,+ in ions has certain competitive characteristics whicls. will be shown in the leaching quantity when H+reacts with electropositive ion in the leaves (Vanek and Draaijers 1994). The nutrient type, nutrient concentration (especially the concentrationof the dissolvable nutrient), contacting time, plant type, and precipitation acidity influence the velocity of nutrient leaching and quantity (Currie 1999). It is generally agreed that in the very beginning of the precipitation, the leaching velocity reaches the maximum, after that it decreases exponentially (Mccammon 1980), so the frequency of precipitation is more important than the intensity of precipitation for the canopy nutrient leaching. But there is no enough evidence to explain whether the dirt on the leaves leads to the exaggerating of the measurement in the beginning of the precipitation. The leaching quantity is different when the nutrient type is different. It is generally considered that the leached N and P are the same. Under the annual precipitation of 3 325 mm, Xu and Hirata (2005) investigated the nutrient leaching in sub-tropical evergreen forest canopy in Okinawa Island in Japan by “miniumbrellamethod” and found that the N and P from the canopy entering to the soil are 43 and 2.6 kg annually, respectively. Staelens et al. (2003) studied the leaching of the electropositive ion from throughflow and stem flow of two deciduous oak forest canopies and found that the leaching quantity of electropositive ion especially Ca2+ and Mg2+had something to do with the study area. The leaching quantity of Ca2+and Mg2+in Chile are 22.5 and 12.2 kg ha-’ yrl and in Flanders are 12.9 and 3.7 kg ha-’ yr’. The reason for leading to the different leaching quantity in the two areas is that the quantity of electropositiveions in soil and leaves is different in those two areas; the quantity of electropositive ions in soil and leaves is significantlyhigher in Chile thanin Flanders. Duchesne et al. (2001) found that in northern Canada there was a lack of N in soil but plenty of N in precipitation, and there was leaching of electropositive ions such as Ca2+,K+,Mg2+and P; forest canopy could assimilate all the N from precipitation, where the assimilation quantity made up to 65% of the maximum
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leaf N storage. He thought that it was the absolute result of the severe shortage of N in the canopy. However, more studies had opposite results: Gallaway and Nadkarni (1991) through “miniumbrella method” found that when compared with the bare land precipitation, the N in TF through canopy increased by 45-60% and the N leaching intensity had significant seasonal changes. Swank and Reynolds (1987) found that compared with BP, NO,--N significantly increased in TF, NO,--N from leaf leaching made up to 82.8%of the total quantity from canopy leaching, and it indicated that NO,--N was the main inorganic N from canopy leaching. Besides plant itself (such as plant reproductiveperiod, nutrient condition, leaf protective tissue) and environmental conditions(temperature, illumination), the canopy nutrient leaching quantity is mainly related to the pH of the precipitation because leaf apoplastic involves the adjustment of cell pH, and the ion moving ability is very different in different pH so that different pH of the precipitation will have significant influence on the leaching result (Mccammon 1980; Sayer and Fahey 1999). Xia et al. (2002) studied the influence of different leaching liquid with different pH, and NaCl stress on the N constitute of the corn canopy leaching materials, and it showed that the quantity of NO,--N, NH,+-N, Org-N, and Tot-N from leaching materials increase with the increase of the plant biomass. The quantity of NH,+-N, NO,--N and Tot-N are higher on leaching with pH 3.5 than pH 7.0, but lower in Org-N quantity. When leaching is carried out using pH 7.0, the N in leaching liquid is mainly Org-N, which makes up 48.3-50.5%,but when using pH 3.5, the N is mainly NO,--N. Leaching at pH 3.5 can increase NO,--N and Tot-N quantity in terms of unit wet weight. N is an essential mineral nutrient in plants, and each type of N quantity from leaching material increases with the increase of the plant growth. It is mainly attributed to the plant growth such as the increase of leaves number and canopy structure, the increase of leachable area, and the accumulation of nutritious matter, as well as, the increase of ion concentration of apoplastic which was caused by the increase of penetration ability of cell membrane, wind injury, and so on. All the factors significantly increase the ion concentration of leaching materials.
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Advances in Nitrogen Loss Leached by Precipitation from Plant Canopy
PROBLEMS AND PROSPECT A progress has been achieved on plant canopy nutrient leaching loss. For example, the nutrient loss during precipitation in forest canopy has been demonstrated; NO,--N has been found to be an important inorganic N type, and so on. But there are still some questions existed to be clarified: (1) The method should be improved. “miniumbrella method” and “ion exchange process” can not precisely differentiatethe canopy nutrient loss which comes from the plant itself, or from the soil and dirt on the canopy surface, so the more precise and reliable methods are needed. (2) Previous researches focused on the study of the forest canopy, but few studies on the farmland plant canopy nutrient leaching loss. Because of fertilization, there is more N, especially mineral N and small molecules of organic N in crop canopy free spaces than in forest canopies. It is indicated that the leaching N source is bigger, but whether the canopy N leaching exists or not needs to be studied. And if the canopy N leaching does exist, it should be studied when researching on N flow and canopy N leaching loss in farmland ecosystem, however, the current research on N flow and canopy N leaching loss in farmland ecosystem does not include this part. (3) If there is a canopy N leaching loss in farmland crops, the factors which influence canopy N leaching loss, such as whether soil N supplement, precipitation period, and precipitationintensityinfluencecanopy N leaching should be further studied. (4)N leaching type should be further studied. The previous researches on N leaching type mainly focused on NO,--N and NH,+-N, and NO,--N was found to be the important inorganic N type, but it is seldom reported whether there is an organic N type, especially, the small molecule solvable N. (5) It is required to research canopy N leaching from plants with different N efficient gene types together. There are fewer reports to indicate whether there is a difference of canopy N leaching for different N efficiency gene types, and whether the canopy leaching is the reason for leading to the decrease of the use efficiency of plant N. (6) No research has existed on discovering the mechanism of the approach and process of canopy N loss, so the theoretical support is lacked on plant canopy N leaching loss. Based on the current research on forest canopy nutrient leaching, free spaces and stomata are
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possibly the main channels of nutrient leaching, and they change with the growth period. Therefore, combining the researches on the leaf area in different growth periods, stomata densities, stomata conductivity, leaf free spaces, and solvable N in the free space, and the research on canopy leaching, is beneficial to further discover the mechanism of canopy N leaching loss and provide theoretical support on canopy N leaching loss. However, there are not so many researches about that currently. Therefore, there is an urgent need to carry out the relevant researches on farmland crop canopy N leaching, find out other approaches of farmland crop canopy N loss except the loss by evaporating in terms of gas, and provide a scientific basis on further understanding plant canopy N loss.
Acknowledgements This work was supported by the National Natural Science Foundation of China (30670326, 3057 1116), and the Talent Program in West China of Chinese Academy of Sciences (2006LH01).
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