Aerobic mineralization and recovery of nitrogen as affected by soil organic matter status in three sites in Kenya

THE SOIL SCIENCE

SOCIETY

OE EAST AFRICA

PROCEEDINGS

OF THE 21ST

ANNUAL CONFERENCE

1st

-

5th December, 2003,

Eldoret, Kenya

THEME:

.

Capacity Building for Land

Resource Management to Meet

the Challenges

of

Food Security in Africa

Editors:

D.N. Mugendi, G. Kironchi,

P.T. Gicheru, C.K.K. Gachene,

AEROBIC MINERALIZATION AND RECOVERY

OF NITROGEN

KENYA

1M.W. Karunditu, D. N. 'Mugendi, IJ.Kung'u, 2B. Vanlauwe 'Kenyatta University, P.O Box 43844, Nairobi, Kenya "Tropical SoilBiology and Fertility, P.O Box 30677, Nairobi, Kenya

Author email for correspondence: karunditum@yahoo.co.uk

ABSTRACT

Pressure on available land has increased due to high population density and other competing land-use demands which haveled to reduced use of traditional fallows and subsequent decline in soil fertility. Aerobic

Nmineralization potential as affected by soil organic matter status in the soil was determined by a 56 day aerobic incubation at 25°C. Calliandra treatment at Maseno and Embutrials had the highest mineralization over the incubation period. This was attributed to its low quality (slow decomposition and N release) and its ability to build up soil organic matter (SOM) in the soil.Recovery of nitrogen in maize plants was also determined and recovery was highest atEmbu trial followed by Maseno trial while Kabete trial had the lowest recoveries. Rainfall distribution and resource quality were the main factors affecting recovery ofnitrogen at the three trials.

Key words: Mineral N, decomposition, organic resources, immobilization

INTRODUCTION

Declining soil fertility is a major problem facing small-scale farming in sub-Sahara Africa(lkerra et al., 2001). InKenya, there is anaverage net mining of 42 kg N, 3 kg P and29 kg K ha' y.1from the soil (Smaling, 1993). As land pressure increases due to increasing population and other competing land-use demands, long duration of traditionalfallows is nplonger a viableoption (Alfred et al., 1996; Ikerra et al., 2001).

Overcoming the limitations in maize based cropping systems of resource-poor farmers in Kenya highlands will result in greatest use efficiency of the resources and increase maize yields from 1520 kg/ha to 3990 kg/ ha/crop (Woomer, 1992).

Soil organic matter plays a major role in sustainable agricultural development by enabling soil to perform efficiently its primary function of supporting plant growth (Dang and Klinnet, 2001; Katyal et al., 2001). It regulates or ameliorates numerous environmental constraints to crop productivity through mineralization and decomposition (Woomer et aI., 1994). Maintenance of SOM therefore is of great importance (Kang and Van der Heide, 1985; National Research Council, 1993). Its functions vary from nutrient supply, water retention, soil structure maintenance and carbon sequestration (Craswell and Lefroy, 2001; Keulen, 2001; Merckx et al.,2001).

Soil is often incubated under controlled conditions to assess its capacity to mineralize and to define the N mineralization potential (Dendooven et al., 1995). Knowledge about the aerobic N mineralization potential is important in crop production because nitrogen is the most important nutrient limiting in crop production. The overall challenge is to develop ways of managing organic matter decomposition to optimize short-and long term release of nutrients and the maintenance of soil organic matter (Mafongoya et aI., 1998). Changes in inorganic N reflect the net mineralization of

of 34° 34' E. The mean annual rainfall is 1800 mm distributed in two distinct rainy

seasons; long rains from March to August

and is short rains from September to January. The soil is a Nitisol according to FAO (1990) with 42% clay, 25% silt and 33% sand. Aerobic Mineralization and Recovery of Nitrogen as affected by Soil Organic Matter

organic N, which takes place in the soil (Bernal et al., 1998).

Effective management of N presents a

greater challenge to the farm operator than

does that of any other fertilizer nutrient

because it can enter or leave the soil-plant system by more routes than any other (Olson and Kurtz, 1982). An adequate supply ofN through fertilization is not only associated with vigorous vegetative growth but also it can speed the maturity of crops and improve

both their productivity and farmer's net

income (Tisdale et al., 1990). Therefore, a

study was conducted to (i) assess the effect

of SOM quality/status on aerobic nitrogen

mineralization potential and (ii) effect of

SOM quality/content on fertilizer N recovery in three sites in Kenya.

MATERIALS AND METHODS

Site description

Three experiments, Nitrogen

Management-N1 at Kabete, Phosphorus Manageme

nt-PM1 at Msinde farm,Maseno and the Embu

Hedgerow Intercropping-HI were

conducted in this study.

Nitrogen Management at Kabete in Central

Kenya is sited at 36° 46'E and 01 ° 15'S

and at an altitude of 1650 m above sea level.

The site is located in the semi-humid climatic zone with a total bimodal rainfall of over 970

mm per annum. The soils are quartz trachyte

geological material, and are typical Humic

Nitisols, with moderate amounts of C,Ca,

Mg, and K but low in available P. The

experiment was established during short

rains of 1999.

PM 1 experiment was found in the highlands

of Western Kenya on Msinde farm near

Maseno. The experiment was established

during the short rainy season of 1995. The

site is located at an altitude of 1420 m above sea level, latitude of 0° 06' N and a longitude

/

TheEmbu experiment was conducted at the

Embu Regional Research Center (RRC),

Eastern Province, Kenya. The center is

located in the Central highlands of Kenya at 0° 30' S, 37° 30' E and at an altitude of

1480 m. The soils are typic palehumults

(Humic Nitiso1s according to

FAO-UNESCO) derived from basic Volcanic

rocks. They are deep, well weathered with

friable clay texture with moderate to high

inherent fertility (Mugendi et al., 1999a).

Total annual average rainfall IS

approximately 1200-1500 mm received in

two distinct rainy seasons: the long rains

(LR) from mid-March to June and the short

rains (SR) mid-October to December. The

experiment was set up in 1992.

Experimental and sampling design

The experimental layout employed RCBD

with three treatments Calliandra

calothyrsus (CC), Leucaena

leucocephala (LL), Tithonia diversifolia

(TD) and a control replicated three times

and the test crop was maize. Soil samples

were collected from 0-10 layer of

experimental plots of the three sites before the onset of 2002 long rains for moisture

determination, and mineral N (NH\ and NO'

3) which enabled the comparison of aerobic

N mineralization potential (within and

between trials).

Plant samples (maize stover, grain and cob) were collected at the end of the cropping season (long rains 2002) from the net plot

of each microplot for the determination of

M. W. Karunditu, et at

determination ofN recovery as affected by

SOMwithin and between trials.

Incubation procedure

Asubsample of airdried soil (200 g) sieved

through a 2 mm sieve from each treatment

was placed into plastic bags and distilled

water added to obtain 45% water holding

capacity (WHC) and preincubated for 7

daysprior toincubation at 25°C after which

dayzero sampling was done. Preincubation

inclosedpolythene bags for7days was done

toexclude anydrying/rewetting phenomena,

prior to incubation (Vanlauwe et aI., 1996).

It was alsonecessary to stabilize and check

on the N mineralization resulting from

microbial activity as a result ofthe wetting

of the soil(day zero sampling). After 7 days

of preincubation 20 g of soil was transferred

into 125ml nalgene bottles for extraction

with 100ml2NKCl and about 25-35 g of

soil were weighed into moisture beakers for

soil watercontent determination. This was

repeated atday7,14,28 and 56 to determine

mineralization potential.

Microplot installation and management

Two microplots measuring 3 m by 1.25 m

had been established in each selected main

plot after day zero soil sampling. They were

surrounded by 25 em tall metal borders

measuring 0.25 x 1.8 m which were inserted

15 em into the soil with 10 em remaining

above the soil surface. to prevent lateral

movement. Labeled ammonium sulphate (5

atom% 15N) and unlabeled ammonium

sulphate (0.3663% 15N)were applied at the

rate of 80 kg N ha' in the ratio of 1:1 split

application to give 40 kg N ha' at planting

and40 kg N ha' at knee height. Phosphorus

and potassium were each applied at the rate

of 100 kg ha' at planting.

Soil analysis (soil extraction

procedures)

For determination of ammonium and nitrate, about 20 g of soil was extracted with 100 ml2 MKCI in 125 ml bottles with shaking for 1 hour at 150 reciprocations min-I and subsequent gravimetric filtration using whatman No. 5 paper prewashed with demonized water. Soil water content was determined on the field moist soils at the time of extraction in order to calculate the dry weight of extracted soil. The extract was used for extractable ammonium using colorimetric method and nitrate by cadmium (Cd) reduction column (Anderson and Ingram, 1993; ICRAF 1995)

Plant analysis

About 1-6 mg of pulverized plant sample to pass through a 1 mm sieve was weighed for organic Nand 15Ndetermination using Automated Nitrogen and Carbon Analyzer-Mass Spectrometer (ANCA-MS) (IAEA, 2001).

Data analysis

Data was analysed using Genstat for windows (version 6) computer package. It was subjected to analysis of variance (ANOVA) for both within site and between sites variations. Treatment means found to be significantly different from each were separated by Least Significant Differences (LSD) at P = 0.05. To determine relationships between SOM content and mineralization potential, the movement ofthe applied fertilizer Nin terms of crop uptake use efficiency relation to N application rate.

RESULTS AND DISCUSSION

Nitrogen mineralization as affected by SOM status

Figure 1 shows mineralization potential from N1-Kabete trial and how SOM status

as a result of immobilization of N in calliandra treatment. Control had thelowest mineralized N throughout the incubation period.

Aerobic Mineralization and Recovery of Nitrogen as affected by Soil Organic Matter

affected the process. At this trial, tithonia treatment had higher mineral N throughout theincubation period compared to calliandra treatment. This observation could have been

45.0

40.0

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.

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-~Calliandra calothyrsus --Control

~Tithonia diversifolia

5.0

a

0.0TI---r--- ..•..

--..,.----,,....--or--- ...•

60

10 20

Incubation days

40 50

Figure I:Nitrogen mineralization potential from Nl Kabete trial

AtHItrial as shown by Figure 2,calliandra

treatment had the highest mineral N

throughout the incubation period followed closely by leucaena treatment. Control had

the lowest mineral N throughout the

incubation period. De Costa and Atapattu (200 I) reported that calliandra has slower biomass decomposition and nutrient release rate which ensures long-term build up ofsoil

organic matter. During the incubation

organic resource quality affected N

mineralization potential (Xu et a!., 1993). The short-long term N capital is the N in the SOM which is mineralized into inorganic N (Giller et a!., 1997). The mineralized N is the available form ofN to the crops during their growth.

~ ::::1

I

I

I

I

~ 80.0. ~.".--- •• ---~--- ...••

~. ~ • -c --calliandra calothyrsus

~ 60.0I" ~--III--__1I1_---...•---,~ --Control

Leucaenaleucocephala

~

Q.) 40.0

c: ~

20.0

10 30

0.0

+

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--_r_--_r_--_r_--_r_--_r_--....

o 20 . 40 50 60

Incubation days

Figure 2: Nitrogen mineralization potential from HIEmbu trial

M. W. Karunditu, et at

Figure 3 shows N mineralization potential

at PM! trial. Calliandra had the highest

amountof mineral Nthroughout the experi -ment except at dayzero where control had

higher amount than calliandra. This could

be attributed to immobilization of N in

calliandra treatment. Tithonia and control

werenot significantly different which could

60

50

C;

_I

:x Z 40

01 E

Z

cu ~₂₀ e ~

10

0

0 10

I

have been caused by immobilization ofN in

tithonia treatment. Low quality resources

mineralize and release N slowly while high

quality resource mineralize and release N

fast. It is through formation of SaM that

organic materials show longer term residual effects on soil which is reflected in the crop

(Palm et aI., 1997).

I

-4111-Caiiiandracalothyrsus ~~Control

~Tithonia diversifolia

30 60

20 40 50

Incubation days

Figure 3:Nitrogen mineralization potential from PM 1 Maseno trial

efficiency. Increased N use efficiency

minimized the opportunity for N loss (Becker

et aI., 1994a; Myers et aI., 1994). Ifrainfall

is adequate and well distributed there is

potential of increasing crop yields with

fertilizer application due to enhanced

recovery (Nyakatawa et aI., 1995). This was

evident at PMl trial in both applications of

Nitrogen recovery as affected by SOM labelled fertilizer.

status At Nl (Figure 5), the first application tithonia

Figure 5shows fertilizer.N recovery at the had the highest uptake while calliandra and

three trials.At PMl trial (Figure 5), the first control were not significantly different. In

application (L) calliandra had the highest the second application calliandra had the

amount of fertilizer N taken up while tithonia highest recovery followed by control while

and control were not significantly different. tithonia had the least recovery. Poorly

In the second application (UL) tithonia and distributed rainfall after the second

calliandra were not significantly different application of fertilizer N.affected uptake

while control had the least fertilizer N and use efficiency which was reflected in

recovery. Calliandra hence used fertilizer N thelow % fertilizer N recovery in the second

efficiently compared to other treatments due application of fertilizer N which was less

to its ability to build up SaM. Good saM than 5%. Inadequate water limited the

status in this trial enhanced fertilizer N use response of crops (Jama et aI., 1995). At

243 The content of N mineralized during the

incubation was higher in the treatments in

both Embu and Maseno trials. The aim of

nitrogen (N) mineralization is to synchronize

mineral N(nitrate) release and crop uptake

to avoidlosses (Cooke, 1980; Owens et aI.,

Aerobic Mineralization and Recovery of Nitrogen as affected by Soil Organic Matter

this trial crop N uptake was restricted by water and apparently little fertilizer was used by the crop hence low recovery (Gentry et al., 1998). A better N uptake in the cropping

system is only obtained by preventing

mineral N,derived from the organic residue,

N fertilizer and from the soil, from being

leached out of the crop rooting zone

(Thomsen and Christensen, 1998).

60.00

PM1 trial

Z _50.00

10.

(!) .!::!

:e

~

~ 30.00

0 co

.

•..

0 20.00

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-10.00

0.00

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, 'V">J ~''V

0~7> ~7>' ~~7> ,'v ~o

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-

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-

<

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~

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(,0

HI trial

,'v N

?:j:-7> ,-0 ,'v. w ,'v N

•7>-0 .,.,7> ~7> ,'V

2;

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Treatments

Figure 5: Recovery offertilizer N from three trials in Kenya

N1 trial

o

Cl

Key

L -Labelled first UL - Labelled

second _~

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Aerobic Mineralization and Recovery of Nitrogen as affected by Soil Organic Matter

high amount was as a result of irrigation which was done at a time when the crucial stage had passed hence it did not benefit the crop leading to poor yields and lower fertilizer N recovery.

The observation made at NI trial can fur

-ther be explained by therainfall distribution pattern in Figure 6 in which Maseno trial received 763.8 mm of rainfall but well dis-tributed over the season, Embu trial 734.1 mm and NI trial 1018.1 mm. At Nl trial the

Rainfall during the long rains 2002 in PM1, N1 and

1200.0

1000.0

E

..§..

ni

1::

ro

0:::

400.0

200.0

0.0

o 20 40 60 80 100 Time (Days)

180

Figure 5 shows the amount of fertilizer N applied that was taken up by the maize plants at HI-Embu tria1. In the first application leucaena had the highest uptake offertilizer N followed by calliandra which was not significantly different from control. In the second application calliandra had the highest % fertilizer N (43.5%) while leucaena and control were not significantly different.

Among the three trials, PMI-Maseno trial used fertilizer N efficiently compared to Kabete and Embu trials. This was attributed to even rainfall distribution throughout the crop growth which enhanced N uptake. At NI trial poor rainfall distribution contributed significantly to low fertilizer N recovery especially in the second application. Lack of adequate water constrained the expected output. Although the soil contains a large pool of soil organic N, it is presumed to be released slowly to satisfy the needs of the

HI, Kenya

/

-Embu

120 140 160

maize crop,therefore N fertilizer application becomes inevitable (Gentry et al., 1998).

This release must be in synchrony with plant uptake to minimize N loss through leaching. One of the major constraints to proper management of fertilizers in small scale farming systems is the lack of information on limiting nutrients like N and its high mobility in form of N03-N (Nziguheba et

al.,2002).

CONCLUSION

Soil organic matter is a potential source of soil available N through mineralization since it plays a majorrole in sustainable agricultural development. It was concluded that .treatments with organic resources oflower quality usually have more N mineralized which could be attributed to their ability to

M. W.Karunditu, et al

Rainfall distribution as a key requirement for

crop growth is indispensable and has to be

evenly distributed throughout the growing season. Among the three sites Kabete trial

performed poorly in terms of recovery

because of poorly distributed rainfall.

Calliandra at Maseno trial had the highest recovery due to its ability to build up SaM.

At Kabete the second application was

completely interfered with as a result of

poorly distributed rainfall and recovery was less than 5% fertilizer N. At Embu trial the

second application of 15N fertilizer had

higher recovery with calliandra having the

best performance. It is more evident that

Calliandra is more effective than high quality organic resources if the aim of the farmer

is to build up SaM. .

ACKNOWLEDGEMENTS

The study was part of on-going work by Tropical Soil Biology and Fertility (TSBF) in conjunction with the Kenya Agricultural

Research Institute (KARl). I am grateful

for the financial support received from the

Rockefeller Foundation's Forum for

Agricultural Resource Husbandry. I also

wish to thank staff at the Laboratory for

Soil Fertility and Soil Biology, Katholieke

Universiet, Leuven, Belgium for their

assistance in isotope analysis. Also special thanks to TSBF laboratory technicians for

their assistance in mineral N analysis.

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