ORGANIC MATTER DECOMPOSITION AND
MICRO ARTHROPOD COMMUNITY STRUCTURE
IN CORN FIELDS UNDER LOW INPUT AND
INTENSIVE MANAGEMENT IN GUAÍRA (SP)
G .S . RO D RIG UES; M .A .V . LIG O ; J .L . DE C. M IN EIRO CNPMA/EMBRA.PA, C.P. 69, CEP: 13820-000 - Jaguariúna, SP.
A b stra ct: T he ra te o f organic m a tte r decom position an d the s tru c tu re o f the com m unities o f m ic ro arth ro p o d s w ere com pared betw een tw o corn fields receiving contrasting a g ricu ltu ral m anagem ent practices (low in p u t an d intensive farm in g ). T he ra te o f decom position tended to be h igher in the intensively m anaged field in th e beginning of the grow ing season, b u t d ecreased to a level significantly lower th an the observed in the low in p u t field by the end of the grow ing season. T his suggested th a t the biological com m unity associated w ith the decom position process could be negatively influenced in th e intensively m anaged field. Analyses o f the s tru c tu re of m ic ro arth ro p o d com m unities in dicated differences betw een the two area s. T he m ic ro arth ro p o d populations p resen t in th e intensively m anaged field su ffered a b ru p t decrease in nu m b ers as the season progressed.
Key W ords: Soil fa u n a, organic m a tte r, decom position, agroecosystem s, farm in g system s, low -input a g ric u ltu re
D ECO M PO SIÇ Ã O DA M A TÉRIA ORGÂNICA E ESTRUTURA DA COMUNIDADE DE M ICR O A RTR ÓPO D ES EM M ILH O CULTIVADO DE FOR M A INTENSIVA E CO M BAIXO USO D E INSUMOS EM GUAÍRA (SP)
Resum o: A tax a de decom posição d a m até ria orgânica e a e stru tu ra da com unidade de m icro artró p o d es foram co m p arad as e n tre dois cam pos cultivados com m ilho m as recebendo m anejos distintos, sendo um cam po m anejado intensivam ente e o u tro com baixo uso de insum os. A tax a de decom posição foi m ais alta no cam po intensivam ente m anejado no início da c u ltu ra , m as decresceu p a ra um nível significativam ente in ferio r àquela observada no cam po com baixo uso de insum os ao final da estação. T al tendência sugeriu que a e s tru tu ra da com unidade dos organism os associados ao processo de decom posição poderia e sta r sendo negativam ente influenciada no cam po intensivam ente m an ejad o . A nálises da e s tru tu ra das com unidades de m icro artró p o d es in d icaram que d iferentes com unidades estavam p resentes nos dois cam pos. Âs populações de m icro artró p o d es presentes no cam po sob m anejo intensivo so fre ram queda a b ru p ta em núm eros, sendo p raticam en te elim inadas já no segundo mês de desenvolvim ento da c u ltu ra.
D escritores: F a u n a do solo, m até ria orgânica, decom posição, agroecossistem as, m anejo agrícola
I n t r o d u c t i o n
The region o f Guaíra, in the Northernmost part o f São Paulo State, is characterized by a very intensive agriculture under rapid technological expansion, markedly with the employment o f center- pivot irrig atio n system s. The heavy financial investment involved in such systems calls for very high yields from the irrigated areas, in order to repay the investment. Hence, allied to the high levels o f m e c h a n iz a tio n , th e a g ric u ltu ra l p rac tic e s normally incorporate heavy inputs o f fertilizers and pesticides. H ow ever econom ically sound this agricultural production system may appear, an
evaluation o f its environmental impacts is warranted (Abreu, 1994).
A preliminary qualitative evaluation o f the development o f some crops in several areas under intensive irrigated management indicated problems potentially linked to unbalances in the soil - plant - soil biota relationships. Firstly, large quantities of the organic residues from previous crops were still presen t on the soil in an alm ost intact state, suggesting that the decomposition process might be slow. Secondly, in spite of the intensive control effort, root pathogens were the main agricultural problem in most crops, possibly due to a relative lack o f competition from antagonistic saprophytic
competitors (Chung et a l , 1988). Lastly, it was possible to notice signs o f toxicity in weeds and even crop plants, especially those sprouting at the bottom o f furrows. Possibly, this was occurring as a result o f fertilizers and pesticides being washed down and accumulating on these spots, due to excessive application.
As a result o f these observations it has been hypothesized that unbalances in the dynamics of organic matter decomposition and in the structure o f the soil community could be involved in those p ro b le m s . T h is h y p o th e sis stem s fro m the observations of Santos & W hitford (1981) that soil microarthropods are responsible for a significant control o f microbial populations, and consequently on the process o f decom position. The role of microarthropods in microbial growth control is not a simple function of their numbers or abundance in the soil (which is greatly influenced by pesticide application regime), but results from a myriad of relationships am ong different trophic groups, spanning from saprophagous to fungivorous to nematode and mite predators (Argyropoulou et al. , 1993; Crossley et al., 1989; Moore, 1988). As a consequence o f this intricate food-web and as a result of the very action of the microarthropods on the organic m atter and d irectly on m icrobial b io m a ss, th e m ic ro b ia l p o p u la tio n s can be maintained in the logarithmic phase o f growth, improving the process o f decomposition of organic matter, release o f nutrients, and biological activity in th e so il(A n d re n e ia /., 1988; Santos ei a / ., 1981; Schroth et al. , 1992).
In order to address the basic hypothesis that unbalances in the com munity structure o f soil m ic ro a rth ro p o d s and in the o rg a n ic m a tte r decomposition process were being caused by the in te n siv e m an ag em en t in irrig a te d a re a s , a co m p ariso n betw een an intensively m anaged irrigated field and a low input field was carried out (El Titi & Ipach, 1989). This paper describes the results o f such a comparison.
M aterial and M ethods
Study site: Two neighboring farms which applied different crop management practices were selected in a single soil patch in Guaíra. Two areas (A and B, low input and intensive agricultural management, respectively) were selected, each representing an experimental treatment. By the beginning o f the siunmer growing season, both areas were sown with
com. Area A was characterized by typical low input management, for the main economic activity of its owner was silkworm raising, and pesticides were not used in the property. Area B consisted o f a plot under center-pivot, and the management involved frequent irrigation and heavy inputs of fertilizers and pesticides.
Assessment of organic m a tte r decomposition: In each of these areas a set of 18 litter bags (20 x 20 cm, 1 mm nylon screen, containing 5 g o f dry mixed-leaf litter collected in a fallow field) were laid down under approximately 5 cm o f soil between the crop rows. Two additional six litter bag sets were laid in each area one and two months after the first set, in order to permit monthly estimates in addition to the cumulative estimates provided by the samples initially laid. This sampling schedule is presented in Diagram 1.
After approximately 30, 60 and 90 days, a subset of 12 litter bags were randomly collected from each area (six for the monthly and six for the cumulative estimates) and brought to the laboratory. The content of each litter bag was put into modified Berlese furmels for mesofauna extraction, being the microarthropods fixed in ethanol-glycerin 3:1 (by volum e) m ix tu re . A fter com plete ex tra c tio n (a p p ro x im a te ly 72 h o u rs ) th e c o lle c te d m icroarthropods were m ounted in m icroscope slides, identified and counted.
After extraction, the organic material was d ried at 105°C fo r 24 h o u rs, w eig h ted and incinerated at 600“C for 8 hours for the estimation o f o rg an ic m a tte r d e c o m p o sitio n (S antos & Whitford, 1981). In this method the decomposition rate is expressed directly as percent loss o f organic carbon. These percentages were normalized by arcsin e tra n sfo rm a tio n and the m eans w ere compared by Student’s t tests.
Assessment of m ic roarthropods: M icroarthropod community was computed as the total number of each m icro arth ro p o d o rd e r, total num ber o f collembola and other arthropods, and total number of mites in each family. This arbitrary taxonomic subdivision was selected based on the role and im portance o f each group in o rganic m a tte r d e c o m p o sitio n and m ic ro flo ra - m e s o fa u n a relationship, and position in the soil food web (Crossley et al. , 1989).
The total number of organisms per period per treatment, instead of mean number per sample
N um ber o f litter bags: Sarrp in g dates: 36 12 12 12+12 12 12+12
11
11
29/10 27/11 1/1 20/2Diagram 1 - Sampling schedule for the three months experimental period. Numbers on the top line represent total litter bags laid for both areas A and B (low input and intensive management, respectively). Bold face numbers indicate samples used for the m onthly estim ates of d e c o m p o sitio n , and re g u la r face numbers represent the samples for the cumulative estimates (except for the f ir s t m o n th , co m m o n fo r b o th e s tim a te s ). A rro w s in d ic a te introduction ( i ) and collection ( t ) of litter bags.
unit, was computed due to the very well described patchy distribution o f mesofauna in disturbed soils (Abbott & Crossley, 1982; Santos et al., 1978). The resulting data consisted then of counts o f total m ic ro a rth ro p o d n um bers in each taxonom ic category, for independent samples in relation to fann (or agricultural management), for three distinct periods of time and for two periodical analysis, monthly and cumulative. As the interest of this smdy lays particularly on the effects o f agricultural m a n a g e m e n t (lo w in p u t v e rs u s in te n siv e management), and not in the effects o f time periods, the periods and periodical arrangement were studied independently, isolating only management effects in each comparison. The question to be posed in this comparison may be stated as follows:
"Are relative numbers o f microarthropod in each studied taxa independent o f m anagem ent practices?"
It shall be noted that the question relates to relative numbers (evenness), meaning that even with accentuated drops in the total numbers (as it is likely to occur especially for the intensive management trea tm e n t, due to p estic id e use) the relative proportions are sustained. This problem is typically one o f comparison between observed and expected numbers, so the test extends naturally to this s itu a tio n (S nedecor & C o c h ra n , 1967). The
advantage of such a method is that it allows the construction o f standardized deviations TABLES, so that each deviation (corresponding to each taxonomic category) may be assessed and discussed specifically. Statistically significant y} values in these TABLES indicate the instances in which the relative proportions o f microarthropods differed in a given period, between the two treatments.
In order to address which individual taxa might have suffered deviations from expected numbers, sp e c ific c o m p a ris o n s w e re p e rfo rm e d in standardized deviations TABLES. The relative proportions o f m icroarthropods found in each treatment in each study period were applied in 12
X 2 (taxonomic categories x treatm ent) R x C
TABLES (note that period 1 is the same for both periodic samples). Tests of significance for each comparison were performed for the following null hypothesis:
Ho; The relative proportion of each microarthropod taxa is the same under low input and intensive agriculmral management.
The TABLES were constructed under the assu m p tio n th a t th e n u ll h y p o th e sis h o ld s, considering as expected values the total number of microarthropods in each taxa for both treatments, using the following expression:
Standard deviations = Oij - Eij / SQRT Eij w h ere: O ij is th e o b se rv e d n u m b e r o f microarthropods in each taxa and Eij the expected number assuming Ho. For such a TABLE, a value o f ± 2 is c o n sid ered to be asso cia te d w ith meaningful deviations (Snedecor & Cochran, 1967)
Results and Discussion
The decomposition rates observed in this study for the summer growing season in cultivated com were very high in both treatments, reaching almost 70% loss of organic carbon in one single month. Such high decom position rates are not uncommon for straw material in tropical situations (Lekha et al. , 1989). As can be observed in Figure 1, the rate of decomposition was significantly higher for the intensive agricultural management (area B) in the first month. This effect was probably related to the higher water (due to irrigation) and nutrient availability in this treatment for that period (Andren
■ Intensive
management
• Low input
29/Out
27/N ov
01/Jan
S am p lin g d ates20/Fev
Figure 1 - Organic matter decomposition rate in corn grown under low input and intensive management. Data points in each period showing different letters are significantly different at a = 0 .0 5 .
et al. , 1995). In the second monthly period the two means were no longer different, while in the third month the samples from the low input treatment ( a r e a A) sh o w ed a s ig n ific a n tly h ig h e r decomposition rate. This result could be related to the increasing detrimental effects o f the pesticides applied in the intensively managed field (Kajak, 1989). The same pattern was observed in the samples o f the cumulative periods, with the rate of decom position being higher in the intensively managed field in the beginning, reversing to a higher rate for the low input field by the end o f the study period (Figure 2).
The number o f microarthropods obtained in each treatm ent for each period o f tim e is presented in Figures 3 through 6. It should be noted that the total numbers o f microarthropods are very d iffe re n t, w ith re m a rk a b le d e c reases in the intensively managed field, again probably due to the detrimental effects of pesticides (El Titi & Ipach, 1989). T he re s u ltin g v a lu e s o f fo r the comparisons of community structures between low input and intensive agricultural management for the five experimental periods (three monthly and two cumulative) is presented in TABLE 1.
The null hypothesis is rejected for the first two monthly periods as well as for the two months cumulative samples. By the end o f the crop cycle the differences in relative proportions disappear, as numbers o f microarthropods diminish abruptly especially in the intensively managed field. So, we conclu d e th a t d iffe re n t soil m ic ro a rth ro p o d community structures were associated with low input and intensive agricultural managements during the first two-thirds of the growing season.
A lth o u g h th is in fo rm a tio n m ay be significant in agroecological terms, suggesting that impacts caused to the mesofauna by the agriculmral management can influence higher processes, such as organic matter decomposition, little is available for the understanding o f implicit relationships that could be explanatory. In other words, what kinds of ecological unbalances could have been caused to the structure o f microarthropod corrmiunities as to result in the observed results? Which taxa were influenced the most, and how so?
These questions were addressed by the analysis o f specific standardized deviations in the observed numbers o f microarthropods in relation to the numbers expected if the null hypothesis held.
c;
o
IZI O aS
o u QSampling dates
Figure 2 - Cumulative organic matter decomposition rate in com grown under low input and intensive management. Data points in each period showing different letters are significantly different at a = 0 .0 5 . L a e l a p i d a e D i g a m a s e l i d a e A s c i d a e E u p o d i d a e C u n a x i d a e I m m a t u r e G a m a s i d a O r i b a t i d a S p i d e r s C h i l o p o d a C o l e o p t e r a n L a r v a e P s o c o p t e r a C o l l e m b o l a 3 9 8 6 0 N u m b e r s 8 0 ■ P e r i o d 1 • e m e r g e n c e □ P e r i o d 2 - d e v e l o p m e n t £3 P e r i o d 3 - m a t u r a t i o n 1 7 8 2 2 8 3 1 7 2
Figure 3 - M icroarthropod community structure for three monthly periods in a corn field under low input management.
a
o £ía
§
Laelapidae Digam aselidae Ascidae Eupodidae C unaxidae Imm ature Gam asida Oribatida Spiders C hilopoda Coleopteran Larvae Psocoptera C ollem bola 168 247 20 40 60 Numbers 80100
■ Period 1 - emergence n Period 2 - developm ent E3 Period 3 - maturation 120Figure 4 - M icroarthropod community structure for three monthly periods in a com field under intensive management.
Numbers
Figure 5 - Microarthropod community structure for three cumulative periods in a com field under low input management.
Laelapidae Digam aselidae Ascidae Eupodidae ^ Cunaxidae â
2 Imm ature Gamasida
p Oribatida Spiders Chilopoda Coleopteran Larvae Psocoptera Collembola 168 247 ■ Period 1 - emergence □ Period 2 - developm ent 0 Period 3 - m arnration 20 40 60 Numbers 80 100 120
Figure 6 - M icroarthropod community strucmre for three cumulative periods in a com field under intensive management.
TABLE 1 - Results o f the five tests of significance performed for microarthropod community structure comparison between low input and intensive farming.
Period 1st month monthly 2nd month 3rd month cumulative 2 months 3 months y 2 A. caic_ 585.52 157.79 13.00 171.17 8.195 p-value < 0.005 < 0.005 > 0 .2 5 < 0.005 > 0 .5 df = 11
These deviations are presented in TABLE 2 for the monthly periods and TABLE 3 for the cumulative periods.
The analyses of these TABLES show that the most meaningful deviations occurred in the first period, at the time o f crop emergence. The main contrasts were represented by a higher relative abundance of mites in the intensively managed field, while collembola were more important in the low input farming. Note that oribatid mites represented the prey in the food web o f the high input agriculture (most probably with an important contribution of nematodes, not smdied), and that predacious mites ( la e la p id a e , d ig a m a s e lid a e , c u n a x id a e , and im m ature gamasida) were proportionally more important. In the second monthly period, however, this pattern changed, with total numbers dropping
bluntly in the intensively managed field. Predacious mites become rare in this field, and collembola still more important in the low input management. In the third monthly period most contrasts are no longer important, as the majority o f groups were eliminated from the intensively managed field.
The same general pattern occurred in the cum ulative sam ples, even though in this case oribatid mites grew in relative importance in the low input field as the season progressed. Again, in the third period the differences disappear. This result may in fact be a bias resulting fi-om the skewed character in the data, caused by the extirpation of organisms in the later periods o f the intensive agricultural management treatment, causing the expected values to be closer to zero.
TABLE 2 - Standardized deviations for microarthropod community structure relative to expected independence from agricultural management for three monthly periods.
M icro arth ro p o d P eriod 1
group Low input Intensive
Period 2
Low input Intensive Low input
0.17 0.02 -0.03
0
0.18 0.12 -0.07 0.04 0 0.06 -2.24 0.28 Period 3 Intensive -1.28 -0.140.22
0 -1.27 -0.80 0.47 -0.20 0 -0.38 15.21 -1.91 Laelapidae Digamaselidae Ascidae Eupodidae Cunaxidae I. Gamasida Oribatida Spiders Chilopoda Coleoptera L. Psocoptera Collembola -1.26 -2.06 0.74 0.49 -4.35 -1.99 -8.15 -0.89 0 -2.01 -0.89 3.84 2.52 4.15 -1.49 -0.99 8.75 3.99 16.40 1.80 0 4.04 1.80 -7.73 0.08 0.74 1.77 0.91 0.18 -0.59 -0.98 -0.17 1.85 -2.44 -0.73 10.13 (significant deviations are evidenced in bold face numbers).-0.14 -1.36 -3.24 -1.66 -0.33 1.08 1.80 0.31 -0.30 4.45 1.33 -1.09
TABLE 3 - Standardized deviations for microarthropod community structure relative to expected independence from agricultural management for cumulative periods.
M ic ro a rth ro p o d P eriod 1 Period 2 P eriod 3
group Low input Intensive Low input Intensive Low input Intensive
Laelapidae -1.26 2.52 -2.20 3.28 0.15 -0.59 Digamaselidae -2.06 4.15 0.54 -0.80 0 0 Ascidae 0.74 -1.49 2.15 -3.14 0 0 Eupodidae 0.49 -0.99 0.38 -0.56 0 0 Cunaxidae -4.35 8.75 -0.35 0.52 0.15 -0.59 I. Gamasida -1.99 3.99 -2.92 3.90 0.12 -0.48 Oribatida -8.15 16.40 9.80 -0.24 0.16 -0.64 Spiders -0.89 1.80 -0.62 0.90 0.10 -0.42 Chilopoda 0 0 0.87 -1.86 0 0 Coleoptera L. -2.01 4.04 -1.48 2.16 -0.48 1.96 Psocoptera 0.89 1.80 -1.43 2.08 0.06 -0.24 Collembola 3.84 -7.73 0.45 -0.65 -0.39 1.57
(significant deviations are evidenced in bold face numbers)
These results show that while all groups were affected in the intensively managed field, the organism s closer to the base o f the food web (especially collembola and oribatid mites) suffered the m ost. As it w ould be expected, w ith the elimination o f the base o f the trophic pyramid the whole community failed.
Even though the results obtained in this work permit only the drawing o f conclusions in terms o f association and not causation, since the data are essentially observational, the objectives p ro p o s e d w ere a c h ie v e d . T he m e th o d was sufficiently malleable to allow the assessment of specific relationships, with clear visualization of
direction and degree o f deviation, besides testing the hypothesis proposed. Finally, it was possible to obtain some insights for the design o f further experiments to be conducted in order to show relationships o f causation in soil microarthropod community structure and agricultural management. Two important conclusions can be obtained from this analysis:
1 - Manipulative and replicated experiments are needed to p erm it in feren ces about causation relationships for agricultural m anagem ent and microarthropod community changes in general, not only for the smdied situation.
2 - Sample units should be individualized so that tests o f significance are not biased by the skewed character o f the data set.
In conclusion, this work indicated that distinct m icroarthropod com m unity structures (represented by relative proportions among taxa, o r e v e n n e s s) a re a s s o c ia te d w ith d if fe r e n t agricultural management, and that an unbalanced com m unity structure reflects negatively in the process o f organic matter decomposition (El Titi & Ipach, 1989). Furthermore, we suggest that the rate of organic matter decomposition and analysis of m icroarthropod communities are good indicator parameters for the assessment o f the environmental impacts in agriculmre.
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Recebido para publicação em 17.07.96 Aceito para publicação em 25.04.97
