Photodegradation for Nutrient Management in the Dry Tropics

doi:10.4236/ojss.2012.21003

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Open Journal of Soil Science, 2012, 2, 17-19

http://dx.doi.org/10.4236/ojss.2012.21003 Published Online March 2012 (http://www.SciRP.org/journal/ojss) 17

Photodegradation for Nutrient Man agement in the Dry

Tropics

Bente Foereid1,2

1Department of Crop and Soil Science, Cornell University, Ithaca, USA; 2Present Address: Simbios, University of Abertay, Dundee,

UK.

Email: [email protected]

Received December 9th, 2011; revised January 10th, 2012; accepted January 19th, 2012

ABSTRACT

Nutrient limitation in agriculture is a big problem in many tropical areas. Much research has focused on the optimal use

of crop residues in agriculture. Recent work indicates that plant residue decomposition rates, including nutrient release

rates can be increased by pre-exposure to sunlight. This has potential as a technology to manipulate nutrient release

from residues to match crop demands. The possible development and use of this technology is discussed. There is some

evidence that this could increase emissions of trace gases, but the increase is thought to be small. Research needs are

discussed.

Keywords: Photodegradation; Tropical Agricu lture; Nutrient Management; Greenhouse Gas Balance

1. Introduction

In many tropical regions, particularly Africa, mineral fer-

tilizers are too expensive to be used extensively by most

farmers. Efficient use of organic residues as fertilizers is

therefore important, and there is a large literature on this

topic [1,2]. Particularly high quality plant litter and resi-

dues that can release most of their nutrients during peak

crop growth are valuable. Attempts to mix different types

of residues to manipulate nutrient release rate to much

crop demand as closely as possible have been met with

varying success.

Several recent studies have indicated that photodegra-

dation, enhancement of decomposition rate caused by ex-

posure to light, may play an important role in plant resi-

due decomposition and in the carbon cycle in semi-arid

ecosystems where other climatic factors appear less im-

portant [3-9]. These environments are characterized by

high radiation levels coinciding with low vegetation cover

for at least part of the year, meaning that the degrading

litter on the soil surface will be exposed to high radiation.

The mechanism for how photodegradation happens is still

imperfectly known, but our earlier experiment indicates

that photodegradation may work by increasing subsequent

microbial decomposition rate in the wet season [10]. Sub-

stantial photo-exposure appears to be necessary, as the

effect was not found when plant litter was exposed to lo-

wer amounts of radiation [10-12]. The mechanism for how

photo-exposure affects litter mass loss is insufficiently

known, but recent work suggests that it particularly af-

fects the degradation of lignin, the plant compound most

resis tant to mi crobia l degrad ation [13]. The effec t of ph ot o-

exposure on nitrogen mineralization has been less studied,

but there is some evidence that this may increase rate of

nitrogen as well as carbon release from the litter substan-

tially [5,10]. In particular, the evidence suggests that photo-

expose can make at least some of the nitrogen in plant lit-

ter mineralize very quickly [10]. It is possible that this is

related to degradation of lignin as lignin is an important

compound in the plant cell wall, and degradation of this

wall could make more easily degradable and nitrogen rich

cell components leak out. Not much is known about how

photodegradation is affected by other environmental fac-

tors. However, there is some evidence that wet litter is

broken down faster when exposed to light [14]. Foereid

et al. [10] also found that temperature alone has some ef-

fect on litter decay rate, but the interaction with light is

not well known.

This poses the question: Can photo-exposure be used

intentionally as a technology to manipulate nutrient re-

lease rate from crop residues to match crop demand? In

this paper I outline this idea, and discuss research needs

to develop the technology.

2. Potential Use of Photodegradation in

Agriculture

Photodegradation has until now only been studied as an

ecological process, and its role in the carbon cycle [4,9].

However, if rate of nitrogen release can be manipulated

Photodegradation for Nutrient Management in the Dry Tropics

by previous exposure to sunlight, this gives new oppor-

tunities timing nitrogen release from crop residues and

other organic material to crop demand in agriculture. This

will be the case everywhere there is a marked dry season

and cropped fields are mostly bare and can be used to

spread the residues. Furthermore, this offers an opportu-

nity to make “low quality” litter that normally will not

release much of its nutrients during the cropping season

more useful. Photodegradation could be taken advantage

of in agriculture by leaving the residues on the ground

for the required amount of time, and them digging them

under. How long exposure that is required for various

sorts of residues would have to be calibrated through ex-

perimentation. However in general it would be expected

that the more the nitrogen release rate needs to be in-

creased to match crop demand, the longer the exposure

needed, as our earlier experiment showed that the short-

est exposure time changed nitrogen release rate much less

than the longest exposure time [10].

There is a need to try out th is system in the field to as-

sess if the benefits are substantial, and the system needs

to be optimized. If a simple relationship between amount

of radiation received and nutrient release rate can be found,

the optimal exposure time only needs to be tested in a

few sites and predictions can be made for other sites. Pre-

vious work has shown that a fairly consistent relationship

between radiation exposure and carbon mineralization

can be developed [9]. There is also evidence that the ef-

fect of photo-exposure depends on lignin content [13],

meaning that it may also be possible to generalize be-

tween litter types. The effect of wavelength distribution

is imperfectly known [13], and the length of photo-ex-

posure may have to be calibrated separately for areas with

very different wavelength distributi on.

In addition, potential drawbacks need to be assessed.

One of them is the greenhouse gas balance of the system.

Primarily methane release needs to be quantified. There

is now ample evidence that both live and dead plant ma-

terial emit low amounts of methane when exposed to UV

radiation [15-17]. This emission is likely to be fairly small,

but still needs to quantified. In addition, Foereid et al.

[10] showed that nitrous oxide emission increases after

photo-exposure in an incubation experiment. It is likely

that th is can be pa rtly or co mplete ly mi tigat ed when plants

are present to take up the available nitrogen (indeed that

is the purpose of the system). However, careful quantifi-

cation of the greenhouse gas balance of the system, as well

as assessment of mitigation options is needed.

This idea points to research needs of both basic and

applied nature. The mechanism of how photo-exposure in-

creases the release rate of nitrogen is not well understood.

It is not merely following the increased carbon release as

the timing of nitrogen release is different [10]. This can

be studied in laboratory incubations. It also needs to be

assessed how photodegradation interacts with tempera-

ture and moisture. To make use of the process in agri-

culture field experiments under realistic conditions are

needed. The system needs to be optimized under realistic

field conditions. Th e technology will only viable in areas

where agricultural fields are left bare in periods with high

radiation, i.e. areas with a dry season. Still there will be a

range of variation in other climatic factors (temperature

and moisture) within those areas, and their interaction

with photodegradation needs to be assessed. The green-

house gas balance of the system also needs to be assessed,

as well as potential mitigation optio ns.

3. Conclusion

Taking advantage of the effect of photo-exposure on nu-

trient release rate has potential for use in agriculture. The

exact amount of time litter needs to be exposed to sunlight

needs to be calibrated, as well potential effects on green-

house gas emissions.

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