Organic Matter Characteristics and Nutrient Content in Eroded Soils CARLOS GARCIA TERESA HERNANDEZ* ASCENSION BARAHONA FRANCISCO COSTA Department of Soil and Water Conservation and Waste Management Centro de Edafologia y Biologia Aplicada de Segura CSlC. P.O. Box 4195 30080-Murcia, Spain ABSTRACT / Twenty-one severely eroded soils of SE Spain (Torriorthent xeric soils) were studied. These soils form a fragile system characterized by soils with a low density of
Given the long period of time necessary for its formation, soil might be considered as a slowly renewable natural resource. A soil reaches a steady state with envir o n m e n t a l factors a n d acquires properties that m a i n t a i n its productive capacity (L6pez-Bermudez and Albaladejo 1990). However, not all factors are favorable to the m a i n t e n a n c e of the steady state, a n d sometimes h u m a n intervention (unsuitable agricultttral practices, the a b a n d o n m e n t of agricultural soils, etc.) can contribute to the deterioration of a soil's natural productivity, as can unfavorable natural factors such as climate (Arnold a n d Jones 1989, Dick 1992). As examples, one can p o i n t to the large land area a b a n d o n e d many years ago for agricultural use. A key factor in soil degradation in this area is the destruction of plant cover, which m e a n s that water erosion a n d salinization processes also play a role, aggravating still further the effects of the semiarid climate. In these soils, the loss or d i m i n u t i o n of organic matter as a c o n s e q u e n c e of p l a n t cover degradation is an added effect (Francis a n d T h o r n e s 1990). Larson (1981) cites soil erosion as the principal cause of the loss of organic matter a n d n u t r i e n t s in soils, which undoubtedly contributes to the loss of productivity. According to Follet a n d Westfall (1986), organic matter is c o n c e n t r a t e d at the surface where it is extremely susceptible to loss through erosion. Because organic
plant cover (<5%), are loamy and occur in a semiarid climate. The soils formerly were used for agricultural purposes but were abandoned at least 15 years ago. These eroded soils had a low total organic carbon content, and their humic substances, humic acid carbon, and carbohydrates were lower compared with soils that had never been cultivated (natural soils). The variables in which the effects of erosion were particularly noted were those related with the active organic matter (respiration and watersoluble organic matter). Those eroded soils with higher salt content showed lower organic matter and carbohydrate contents. Only total nitrogen was correlated with the carbon fractions in the eroded soils.
matter constitutes a reserve of nutrients, its condition is a good indicator of n u t r i e n t loss in eroded soils. In this paper we d e t e r m i n e a series of variables (basically c o n c e r n i n g organic matter a n d nutrients) in severely eroded 21 soils. They were eroded due to the absence of a stable plant cover a n d their exposure to a semiarid climate. T h e soils are in an extremely fragile sytem that will be difficult to regenerate, but that should be studied for a better u n d e r s t a n d i n g of the processes of degradation that the area u n d e r g o e s a n d to serve as a basis for the d e v e l o p m e n t of suitable methods and techniques to retard or reverse such processes. A comparison of the variables d e t e r m i n e d in this fragile system and the same variables in soils exposed to the same climate but not to h u m a n intervention (with a m e d i u m dense matorral) will p o i n t to the c o n t r i b u t i o n of this h u m a n activity to the degradation process.
Materials and Methods
*Author to whom correspondence should be addressed.
T h e area studied belongs to the SE Spanish Mediterr a n e a n region, within the province of Murcia. The length of the studied area is 100 km, with seven zones b e i n g chosen within it (Figure 1). All the zones have characteristics in c o m m o n , so that they can be referred to as f o r m i n g a fragile system. (1) T h e soils of the zones studied are loamy [ T o r r i o r t h e n t xeric according to the Soil Survey Staff (1994) ] ; as Diaz (1992) a n d Albaladejo a n d D~az (1990) indicated, this lithological substrate is the most liable to erosion of those lying in the Spanish M e d i t e r r a n e a n region; they are very compacted with A-C type profiles a n d a very low organic matter c o n t e n t
EnvironmentalManagementVol. 20, No. 1, pp. 133-141
9 1996 Springer-VerlagNew York Inc.
KEY WORDS: Arid soils; Carbohydrates; Nutrients: Organic matter; Soil degradation;Soil respiration
134
c. Garcia and others
Figure 1. Location of the different zones chosen for soil sampling.
Table 1.
Physical characteristics of upper 15 cm in eroded soils
Zone
Bulk density ( g / c m -:~)
Particle density ( g / c m -:~)
Total porosity (%)
Stable aggregates (%)
Soil
Texture
1
1 2 3
Clay loam Clay loam Clay loam
1.3 1.2 1.3
2.5 2.5 2.5
47.9 50.5 49.3
37.5 37.9 38.3
2
4a 5~ 6~
Clay Clay Clay
1.2 1.2 1.2
2.2 2.4 2.7
45.8 52.3 47.0
45.7 33.4 37.9
3
7 8 9
Silty clay Silty clay Silty clay
1.3 1.3 1.3
2.5 2.5 2.5
47.6 49.6 49.1
18.3 20.9 33.7
4
10~ 11 ~ 12=
Clay Clay Clay
1.2 1.1 1.2
2.3 2.3 2.4
49.9 51.9 52.4
60.5 58.2 53.1
5
13 14 15
Clay loam Clay loam Clay loam
1.1 1.1 1.2
2.4 2.5 2.6
57.1 55.4 53.5
62.9 63.7 64.2
6
16 ~ 17" 182
Clay Clay Clay
1.3 1.2 1.3
2.4 2.4 2.5
48.4 49.1 49.3
40.2 45.1 41.1
7
19 ~ 20 ~ 21 ~
Silty clay Silty clay Silty clay
1.2 1.2 1.2
2.6 2.3 2.4
52.0 48.8 51.0
39.8 53.1 52.5
1.2 0.066 5.5
2.4 0.098 4.1
50.4 2.75 5.5
44.1 12.4 28.0
1.1
2.5
57.8
64.0
X SD CV (%) Natural soils ~Gypsiferous soils.
Clay loam
135
Organic Matter and Nutrients in Eroded Soils
Table 2.
Zone
Organic matter parameters in eroded soils studied ~
Soils
TOC (g/kg)
RC (g/kg)
Ext C (g/kg)
Ext C X 100/ TOC
Humic C (g/kg)
Humic C • 100/ Ext C
OMR (%)
Total carbohydrates (g g l u c o s e / k g )
1
1 2 3
8.07 8.39 10.25
1.35 1.63 1.45
1.75 1.81 2.02
22.46 22.48 20.39
0.57 0.65 0.78
33.64 37.30 39.85
16.7 19.4 14.1
2.13 2.88 2.44
2
4 5 6
5.20 5.51 4.75
0.75 0.81 0.78
0.45 0.48 0.54
10.05 9.99 12.90
0.01 0.01 0.01
2.20 2.76 1.85
14.4 14.7 16.4
0.99 1.25 1.28
3
7 8 9
5.34 5.97 5.98
0.90 1.16 1.27
0.76 0.86 0.87
14.97 15.07 15.31
0.18 0.21 0.19
24.40 25.53 22.76
16.8 19.4 21.2
1.41 1.49 1.97
4
10 11 12
7.62 6.66 7.57
1.14 1.07 1.20
0.87 0.88 1.08
12.70 14.65 15.58
0.17 0.16 0.14
20.98 19.65 14.59
14.9 16.1 15.8
1.89 1.74 1.94
5
13 14 15
17.85 12.82 8.70
1.49 1.17 1.06
4.12 2.56 1.72
24.12 20.85 19.80
1.47 1.09 0.58
37.27 44.52 34.51
8.3 8.9 12.1
4.07 3.25 2.26
6
16 17 18
4.74 4.72 't.40
0.93 0.94 0.86
0.59 0.57 0.42
13.05 12.59 10.06
0.12 0.10 0.07
20.63 19.41 18.34
19.6 19.9 19.5
0.69 0.64 0.22
7
19 20 21
5.03 5.08 4.42
0.92 0.88 0.74
0.52 0.49 (}.54
10.52 10.16 12.74
0.09 0.10 0.14
18.86 20.74 21.84
18.2 17.3 16.7
0.74 0.52 0.66
X SD CV (%)
7.09 3.28 46.20
1.07 25.9 24.6
1.13 0.92 80.65
15.25 4.59 30.1
0.32 0.39 120.2
22.93 16.7 51.1
16.05 3.68 22.9
1.70 0.11 67.2
Natural soils
20.20
4.87
5.81
43.7
24.1
28.7
2.54
6.8
"TOC: total organic carbon; RC: released carbon in 51 days; Ext C = carbon extractable with 0.1 M. pH 9.8, NaaP.,O;;OMR: overall mineralization rate.
Table 3. Cumulative released carbon, k x m values, and slopes of curves corresponding to mineralization of labile (ml) and recalcitrant (m2) carbon Zone 1 2 3 4 5 6 7 Natural soils
Equation C = ki"" C= C= C= C= C= C= C= C=
2.17 t ~'7"" 1.619 t ~''~r'-'z 2.034 t f''~7'~ 2.066 t c'~;'~~ 2.292 t ~u~'':~ 1.971 t ~''~:''':~ 1.948 t t'~'-':~4 20.261 t ''~:~
r
0.9996 0.9997 0.9987 0.9992 0.9988 0.9998 0.9997 0.9912
k •
m
m~ ~'
n~'
m t / r~_,
1.52 1.088 1.375 1.381 1.431 1.246 1.214 12.87
1.60 1.78 2.35 1.86 0.75 2.25 2.09 2.30
0.95 1.16 1.35 1.94 0.41 1.35 1.15 1
1.6 1.5 1.7 1.9 1.8 1.6 1.8 2.3
"c = released carbon in 51 days; t = time; k and m are constants. ~m~= slope of the line for mineralization kinetics of labile C. ' ~ = slope of the line for mineralization kinetics of recalcitrant C.
t h r o u g h o u t t h e p r o f i l e . (2) T h e p l a n t c o v e r o f t h e differ-
15 y e a r s a g o . P l a n t c o v e r is c o m p o s e d m a i n l y o f slowly
e n t z o n e s is v e r y s c a r c e ( < 5 % d e n s i t y ) , w h i c h l e a d s u s
g r o w i n g low b u s h e s ( L y s e u m ,
to d e n o m i n a t e t h e m as b a r e soils. T h e soils w e r e u s e d
w h i c h a f f o r d p r o t e c t i o n to t h e soil. (3) T h e c l i m a t e o f
for agricultural purposes but were abandoned
all t h e z o n e s is s e m i a r i d ( a n n u a l r a i n f a l l < 2 5 0 m m ;
at least
Rosmarinus,
Cistaceae),
136
c. Garcia and others
Figure 2. Cumulative released carbon during the decomposition of the organic matter of soils from zones 1-7 in the incubation experiment.
annual evapotranspiration potential 1000 mm; high m e a n t e m p e r a t u r e 20~ T h e only difference between the zones studied concerns the loam content, zones 2, 4, 6, and 7 b e i n g gypsiferous. Electrical conductivity (EC) is < 2 0 0 IxS/ cm (1:5 solid-liquid extract). In the o t h e r zones the EC values range from 600 to 1200 I~S/cm. T o carry out the present study, three soils in a 2000-m=' plot were chosen from each z o n e (21 soils in total). Each soil sample consisted o f six subsamples taken at a depth o f 0-15 cm. T h e subsamples making up each sample were mixed, sieved to < 2 mm, and stored at 4~ until analysis. T h e slope o f the land from which the samples were taken never e x c e e d e d 2 % - 3 % . T o ascertain how the 21 studied soils ( e r o d e d and with scant plant cover) differed from o t h e r s in the same area that had not u n d e r g o n e h u m a n intervention, the values o f soils supporting Quercus rotundifolia (the natural matorral of the Spanish M e d i t e r r a n e a n region) were taken as reference. Particle density was d e t e r m i n e d by the m e t h o d pro-
Table 4. Content of water-soluble carbon (WSC), carbohydrates (WS carb), phenols ONS phenols), and proteins (WS proteins)in soils studied Zone
Soil
WSC (mg/kg soil)
WS carb (mg glucose/kg soil)
WS phenols (mg/kg soil)
WS proteins (l~g albumin/kg soil)
1
1 2 3
72.15 71.28 95.85
38.91 43.62 42.44
20.39 22.90 22.57
14.39 15.65 17.73
2
4 5 6
23.35 26.53 25.23
33.98 44.55 27.07
7.51 7.12 8.03
ND~ ND ND
3
7 8 9
59.90 43.53 45.83
51.31 39.37 41.00
8.59 8.58 8.81
5.65 5.44 5.66
4
10 11 12
28.49 25.08 23.26
46.94 52.94 50.24
6.41 6.77 6.91
5.74 0.64 4.17
5
13 14 15
105.44 72.67 63.24
94.90 50.78 48.16
14.51 15.53 12.61
13.03 8.09 6.18
6
16 17 18
32.84 34.94 22.85
37.47 39.77 36.37
4.74 8.02 9.43
4.00 4.30 5.35
7
19 20 21
38.15 25.30 17.47
37.83 41.17 39.27
4.09 10.88 7.68
1.99 4.14 2.63
X SD CV (%)
45.39 25.70 56.70
44.66 13.60 30.50
10.85 5.6 51.7
5.79 4.6 80.6
Natural soils aND = not detected.
294
145
34
--
Organic Matter and Nutrients in Eroded Soils
Table 5. Correlations between organic matter parameters in eroded soils studied a Parameters TOC-Ext C TOC-humic C TOC-RC TOCM)MR TOC-WSC TOC-WS carb TOC-T carb Ext C-humic C Ext C-RC Ext C_,-OMR Ext C-WSC Ext C-WS carb Ext C-T carb Humic C-C-CO,, Humic C-OMR Humic C-WSC Humic C-WS carb Humic C-T carb C-CO,z-OMR C-CO,z-WSC C-CO._,-WScarb C-CO,,-T carb OMR-WSC OMR-WS carb OMR-T carb WSC-T carb
r
P
0.9812 0.9573 0.6116 - 0.7657 0.8160 0.7857 0.9551 0.9841 0.6574 -0.6986 0.8785 0.7398 0.9524 0.6206 -0.6938 0.9000 0.6588 0.9122 -0.7016 0.4774 0.7176 - 0.4904 - 0.5258 -0.6623 0.5004
0.0000 0.0000 0.0032 0.0001 0.0000 0.0000 0.0000 0.0000 0.0120 0.0004 0.0000 0.0001 0.0000 0.0027 0.0005 0.0000 0.0120 0.0000 -0.0004 0.0286 0.0003 0.0240 0.0140 0.0011 0.0209
"TOC: total organic carbon; Ext C: pyrophosphate extractable carbon; OMR:overallmineralizationrate; WSC:water-solublecarbon; WScarb: water-soluble carbohydrates;T carb: total carbohydnltes.
Figure 3. Plot of cluster for the 21 soils studied.
137
posed by the Spanish G r o u p of Analytical Method Standardization (1978), bulk density was d e t e r m i n e d on soil clods by using paraffin as clod impermeabilizer (Barah o n a and Santos 1981), a n d the percentage of stable aggregates in samples sieved to 0.2-2 m m following the m e t h o d described by Lax and others (1994). T h e m e t h o d of Yeomans a n d B r e m n e r (1989) was followed to ascertain the total organic carbon c o n t e n t of the soils (TOC), the humic substance carbon (extracted with 0.1 M Na~P,,OT, pH 9.8, in a solid-liquid ratio of 1:10), and the humic acid carbon (precipitated with HoSO4 from the previous pyrophosphate extract at p H 2). Total carbohydrates were measured using the m e t h o d of Brink a n d others (1960). The respiration e x p e r i m e n t used 50 g of soil moistened to 65% of its water h o l d i n g capacity in hermetically sealed flasks, which c o n t a i n e d a vial with 10 ml of 0.5 M NaOH. T h e flasks were i n c u b a t e d for 51 days at 28~ with the N a O H b e i n g evaluated every five days with 0.1 M HCI. Using an aqueous extract of the soils (1:5 solid-liquid ratio), the following were determined: water-soluble carb o n (by oxidation with K..,Cr,.,O7 and measure of absorbance at 590 nm), soluble carbohydrates (Brink and others, 1960), and the polyphenols detected by Folin's reagent a n d water soluble proteins (Rad I992). Total nitrogen was measured by Kjeldhal's method; N-NH~ + extracted with 2 M KCI was d e t e r m i n e d with an NH~+ selective electrode; N-NO3- in an aqueous extract was measured by difference of absorbance in ultra-
138
Table 6. Zone
c. Garcia and others
Macronutrient content in eroded soils Soil
Total N (g/kg)
N-NO:C (mg/kg)
N-NH.~§ (mg/kg)
Total P (g/kg)
Ext P (mg/kg)
Total K (g/kg)
Ext K (mg/kg)
1
1 2 3
0.80 1.03 1.07
3.6 4.0 3.0
14.5 19.7 13.5
0.20 0.16 0.23
10 15 11
2.43 2.23 3.31
168 104 156
2
4 5 6
0.52 0.68 0.40
3.2 2.2 2.9
7.0 10.0 8.9
0.17 0.25 0.26
ND ND ND
6.59 7.58 6.34
250 204 281
3
7 8 9
0.52 0.69 0.80
4.7 8.0 4.4
5.6 7.2 4.8
0.67 0.76 0.69
9 11 11
7.49 9.52 7.97
278 319 274
4
10 11 12
0.86 0.60 0.76
6.6 2.1 1.0
50.8 28.9 20.8
0.31 0.28 0.23
2 ND ND
8.06 7.66 5.38
255 238 187
5
13 14 15
1.15 0.91 1.14
1.3 3.9 1.6
17.7 13.6 4.8
0.39 0.42 0.36
12 12 11
3.24 5.00 3.71
145 134 115
6
16 17 18
0.42 0.50 0.50
3.0 1.3 2.4
7.3 5.9 8.6
1.00 0.98 0.91
10 8 9
10.63 10.28 9.85
335 319 283
7
19 20 21
0.48 0.46 0.42
0.9 0.0 1.2
5.4 8.1 5.8
0.79 0.88 0.68
11 11 9
9.64 9.84 10.00
341 333 389
0.70 0.25 35.6
2.7 2.0 73.5
12.8 10.8 84.3
0.50 0.29 58.4
8 5 64
6.98 2.78 39.80
243 83 34.3
14
4
0.59
ND
2.4
X SD CV % Natural soils
1.12
469
"ND: not detected.
violet light at 210 a n d 270 n m (Bolarin a n d others 1982). Total p h o s p h o r u s a n d potassium were measured directly in the nitric-perchloric digestion of the material; total phosphorus a n d NaHCO3-extractible phosphorus (Olsen 1954) by the m e t h o d of Murphy a n d Riley (1962), a n d total potassium a n d a m m o n i u m acetate-extractible potassium by flame photometry. T h e analyses were carried out in triplicate, and the data were submitted to statistical analysis (standard deviation, coefficient of variation, correlation matrix, a n d cluster plot).
Results and Discussion Some of the physical characteristics of the soils are shown in Table 1. T h e 21 soils are very similar as regards density a n d total porosity (coefficient of variation about 5%), although the percentage of stable aggregates in these eroded soils varies considerably (28% CV). T h e sample of the natural soil studied showed better physical characteristics than the m e a n of the 21 eroded soils. T h e TOC values of 18 of the 21 soils were below 10
g / k g while the m e a n for this variable in the eroded soils was 7.1 g/kg, which is practically a third of the natural soil's value (Table 2). This lends weight to the idea that the absence of plant cover a n d the c o n s e q u e n t erosion to which the soils are exposed results in a very low organic matter content. Only part of this T O C in eroded soils was extractible with sodium pyrophosphate (X = 15.25%) a n d belongs to the h u m i c substance C, which suggests that most of the T O C forms part of the h u m i n s and corresponds to a very inactve organic matter. In natural soils, u n d i s t u r b e d by man, almost 30% of organic carbon belongs to h u m i c substances (Table 2). O f the seven zones studied, two (zones 1 a n d 5) showed considerably higher T O C a n d extractable C values. T h e values of the humic acid C in the e r o d e d soils studied range from 0.01 to 1.47 g / k g and, in the carbon fractions d e t e r m i n e d by us, show the highest variation coefficients (120%). Because n o t all the carbon extracted comes from humic acids (the m a x i m u m value detected was 44% of soil 14), o t h e r c o m p o u n d s such as fulvic acids, carbohydrates, etc., must exist in the
Organic Matter and Nutrients in Eroded Soils
humic substances extracted that take part in the mineralization a n d humification processes. However, d u e to the climatic conditions to which these soils are e x p o s e d (very long dry periods), these processes are n o t very active. Therefore, the turnover o f organic m a t t e r is low. T h e m e a n value of C released as CO2 (RC) d u r i n g the 51 days of the respiration e x p e r i m e n t was 107.1 m g RC/100 g soil (Table 2), a value that is very low comp a r e d with that d e t e c t e d in the natural soil (487 m g RC/100 g soil). O n c e again, the low reactivity of the organic m a t t e r c o n t a i n e d in these e r o d e d soils is d e m onstrated; the absence of p l a n t cover has a negative effect on biological activity. This result was expected, because studies in the same soils showed very low values for biomass C (dehydrogenase) related with microbiological activity (Garcia a n d others 1994). All the above suggests that these soils are best r e g e n e r a t e d by the addition o f labile organic fertilizers that are capable o f activating the soils' microorganisms and triggering the processes of mineralization a n d humification of the organic matter (Dfaz 1992, Garcia and others 1992). Organic m a t t e r was mineralized similarly in all the e r o d e d soils. T h e RC fits a potential equation o f the type C = kt", where C is the RC, t is the ler.gth o f the e x p e r i m e n t , a n d k a n d m are constants (Figure 2). T h e correlation coefficients o f the above equations are shown in Table 3. W h e n a similar fit is m a d e for the RC in the natural soils, the products k X m, which indicate the microbiological activity of the soils (Pal a n d B r o a d b e n t 1975), are m u c h greater than in e r o d e d soils. W h e n the log o f residual C fraction (C residual = TOC RC • 1 0 0 / T O C ) as time elapses is r e p r e s e n t e d graphically, two kinetics are observed in all the soils r e p r e s e n t i n g mineralization of the most labile substrates followed by that of the most recalcitrant substrates. During the first, relatively labile substrates are m i n e r a l i z e d and d u r i n g the second, the m o r e recalcitrant substrates are mineralized. For this reason the slopes for the first process (m~) were h i g h e r than those of the s e c o n d (me) in all the soils (Table 3). This p h e n o m e n o n has b e e n observed by o t h e r authors (Durall and Parkinson 1987). A similar behavior was seen in the natural soils, a l t h o u g h the presence o f two kinetics is even m o r e m a r k e d , as can be seen by the h i g h e r value of the q u o t i e n t o f the slopes o f the straight lines (Table 3). T h e lowest values for overall mineralization rate (defined as: O M R = C~ 1 0 0 / T O C , where Ct = RC in 51 days), c o r r e s p o n d e d to those of zone 5, in which the highest T O C values were r e c o r d e d (Table 2). This suggests that in these soils, the percentage o f RC c o m p a r e d with T O C is not c o r r e l a t e d with the T O C c o n t e n t o f the soil. T h e m e a n values o f total carbohydrate c o n t e n t (Ta-
139
ble 2) in the e r o d e d soils was 1.70 g / k g soil, while in the natural soils it reaches 6.8 g / k g soil. S o m e authors suggest a correlation between the p e r c e n t a g e o f stable aggregates a n d the c a r b o h y d r a t e content, since the latter can act as cementing agents (Cheshire a n d others 1983). Others mention the existence in soils o f stable microaggregates f o r m e d by the b o n d i n g o f the clay a n d h u m u s (Fortun and others 1989). However, in the soils studied in this experiment, we observed no correlation between the percentage o f stable aggregates a n d the carbohydrate or humic acid content. In these e r o d e d soils, it was probably the p o p u l a t i o n of fungi a n d actinomycete hyphi that have a d a p t e d to the prevailing soil conditions and contribute to the a m o u n t o f stable aggregates. T h e water-soluble organic m a t t e r fraction is o f great i m p o r t a n c e in soil solutions (Kniters and M u l d e r 1993) a n d can act as indicator o f the products available for mineralization (Cook and Allan 1992). In it, we identified a fraction of water-soluble carbon, m o n o - a n d polysaccharides, polyphenols, a n d some water-soluble proteins (Table 4). T h e water-soluble carbon (a fraction that can contain part o f the fulvic acids) a n d the polyphenols a n d soluble p r o t e i n c o n t e n t were highest in zones 1 and 5. T h r o u g h o u t the study, zones 1 and 5 differed considerably from the other zones, having a g r e a t e r c o n t e n t of TOC, Na~PeO,-extractible C, humic acid carbon, carbohydrates, etc. Perhaps their greater organic matter a n d carbon fraction c o n t e n t a n d the h i g h e r activity of this organic matter (higher k • m values, T a b l e 3, a n d h i g h e r water-soluble organic m a t t e r content) m e a n that these soils are less d e g r a d e d . Moreover the same soils showed the lowest electrical conductivity ( < 2 0 0 ixS/cm) while in the o t h e r soils EC ranges from 600 to 1200 i~S/cm due, in particular, to the existence of gypsiterous marls. In previous studies in the same Spanish M e d i t e r r a n e a n area (Garcia a n d o t h e r s 1994), we d e t e c t e d a lower degree o f microbiological activity in soils showing high electrical conductivity. This, too, suggests that the soils in the zones o f least salinity show the least biological degradation. T h e soluble carbohydrates d e t e c t e d in the e r o d e d soils along with the o t h e r p a r a m e t e r s o f the water extract are much less in evidence than in the n a t u r a l soils. A c c o r d i n g to De Luca a n d Keeney (1993), the carbon that is soluble in a n t h r o n e reflects the level of free sugars associated with p l a n t residues available to the microorganisms and t h e r e f o r e reflects microbial activity. This again confirms that a principal effect o f erosion involves the loss of labile organic matter, which serves as an energy, source for the microorganisms. T h e r e was a clear correlation between the variables
140
c. Garcia and others
Table 7. Range of some parameters determined in eroded soils from the Mediterranean area a Parameter Total organic C (g/kg soil) Humic substances C (g/kg soil) Humic acid C (g/kg soil) Total carbohydrates (g glucose/kg soil) Water soluble C (mg/kg soil) Water soluble carbohydrates (rag glucose/kg soil) Total N (g/kg soil) Total e (g/kg soil) Total K (g/kg soil) Extractable N (NH4§ + NO~) (mg/kg soil) Extractable P (mg/kg soil) Extractable K (mg/kg soil)
Value range (mg/kg) 4.40-7.62 0.42-1.08 0.01-0.21 0.22-1.97 17.5-59.9 27.1-52.9 0.40-0.86 0.17-1.00 5.38-10.6 6.3-57.4 0-11 187-389
"Based on soils of zones 2, 3, 4, 6, and 7.
c o n c e r n i n g carbon in the eroded soils (Table 5). T h e highest correlation coefficients were shown by the most stable carbon (TOC, h u m i c substance C, humic acid C a n d total carbohydrates). T h e lowest values, although always with P < 0.03, b e l o n g e d to the most labile carbon fractions (C released as CO,, a n d soluble carbohydrates). All the correlations between variables were positive, except where the overall mineralization rate was o n e of the variables. These correlations were negative because the biological activity is so low that the fraction of organic matter mineralized is less in soils with more organic carbon. O f all the nutrients e n c o u n t e r e d in the eroded soils (Table 6), total K was the most a b u n d a n t followed by total N and total P. Unlike the parameters c o n n e c t e d with organic matter, the c o n t e n t of these nutrients in the natural soil does not exceed that f o u n d in the eroded soils. O n the other h a n d , the natural soils have a greater NoNO~- a n d extractable K content, which indicates the different degree of mineralization of these nutrients in degraded soils as c o m p a r e d to that which takes place in the natural soils. When the parameters related to organic matter were described, zones 1 a n d 5 were singled out as being less degraded because of their lower salt c o n t e n t and higher organic matter content. As regards n u t r i e n t content, only total N showed a similar behavior to organic matter in the eroded soils, so there was a good positive correlation between total n i t r o g e n a n d TOC ( r = 0.8096, P < 0.001), RC (r = 0.7289, P < 0.0002), water-soluble carbon (r = 0.7841, P < 0.0001), a n d total carbohydrate c o n t e n t ( r = 0.8615, P < 0.0001). Based on all the above data, it is possible to establish
ranges of values for some parameters c o r r e s p o n d i n g to a fragile system of degraded soils formed o n loams, with a semiarid climate, without plant cover a n d high degree of erosion (Table 7). A three-cluster plot (Figure 3) using the organic matter parameters for the 21 soils studied showed the existence of five zones that were highly h o m o g e n e o u s and two zones (1 a n d 5) that were distinct. Table 7 shows the ranges for the r e m a i n i n g 15 soils whose characteristics are similar. In this way comparisons with other ecosystems can be made, using the above values as reference to establish the level of degradation or reclamation of a particular ecosystem.
Literature Cited Albaladejo, J., and E. Diaz. 1990. Degradaci6n y renegeraci6n del suelo en el litoral mediterr~ineo espafiol: experiencias en el proyecto Lucdeme. Pages 191-214 in J. Albaladejo, M. A. Stocking, and E. Diaz (eds.), Soil degradation and rehabilitation in Mediterranean environmental conditions. CEBAS-CSIC, Madrid. Arnold, R. W., and C. A. Jones. 1989. Soil and climate effects upon crop productivity and nutrient use. In Soil fertility and organic matter as critical components of production system. Soil Science Society o f , ~ e r i c a . Special publication no. 19, 166 pp. Barahona, E., and F. Santos. 1981. Un nuevo m6todo para la determinaci6n de densidades aparentes y del coeficiente de extensibilidad lineal (COLE) por el m~todo de la parafina. Anales de Edafologia y Agrobiologia 40:721-725. Bolarin, M. C., M. Romero, and M. Cart. 1982. Determinaci6n de fenoles y formas de N en aguas. Conservaci6n de muestras y tratamiento previo. Anales de Edafologia y Agrobiologia 41:1-10. Brink, R. H., P. Dubar, and D. L. I,inch. 1960. Measurement of carbohydrates in soil hydrolysates with anthrone. So/l Science 89:157-166. Cook, B. D., and D. L. Allan. 1992. Dissoh,ed organic carbon in old field soils: total amounts as a measure of available resources for soil mineralization. Soil Biology and Biochemistry 24:585-594. Cheshire, M. V., G. P. Sparking, and C. M. Mundie. 1983. Effect of periodate treatment of soil on carbohydrate constituents and soil aggregation.Journal of Soil Science 34:105-112. De Luca, T. H., and D. R. Keeney. 1993. Soluble anthronereactive carbon in soils: effect of carbon and nitrogen amendments. Soil Science Society of America Journal 57: 1296-1300. Diaz, E. 1992. Efecto de la adici6n de residuos urbanos en la regeneraci6n de suelos degradados corot medio de control de la desertificaci6n. PhD thesis. Universidad de Murcia, Facultad de Ciencias Biol6gicas. Dick, R. R. 1992. A review: long-term effects of agricultural ~stems on soil biochemical and microbial parameters. Agricultural Ecosystems and Environment 40:25-36. Durall, D. M., and D. Parkinson. 1987. Mineralization potential
141
Organic Matter and Nutrients in Eroded Soils
on surface minespoil of the labile and recalcitrant fractions of ~4C-labelled timothy ( Phelum pratense) litter. ,Soil Biology and Biochemestry 19:43-48. Follett, R. H., and D. G. Westfall. 1986. A procedure for conducting fertilizer recommendation comparison studies.Journal of Agronomy 15:27-29. Fortun, A., C. Fortun, and C. Ortega. 1989. Effect of farm and manure and its humic fractions on the aggregate stability of a sandy-loam soil. Journal of Soil Science 40:293-298. Francis, C. F. a n d J . B. Thornes. 1990. Matorral: Erosion and reclamation. InJ. Albaladejo, M. A. Stocking, and E. Diaz (eds.), Soil degradation and rehabilitation in Mediterranean environmental conditions. CEBAS-CSIC, Madrid. Garcia, C., T. Hernfi.ndez, and F. Costa. 1992. Variation in some chemical parameters and organic matter in soils regenerated by the addition of municipal solid waste. Environmental Management 16:763-768. Garcfa, C., T. Hern;indez, and F. Costa. 1994. Microbial activity in soils under Mediterranean environmental conditions. Soil Biology and Biochemistry 26:1185-1191. Kniters, A. T., and W. Mulder. 1993. Water-soluble organic matter in forest soils. I. Complexing properties and implications for soil equilibria. Plant and Soil 152:215-224. Larson, W. E. 1981. Protecting the soil resource base. Journal of Soil and Water Conservation 36:13-16. Lax, A., E. Diaz, V. Castillo, andJ. Albaladejo. 1994. Reclamation of physical and chemical properties of a salinized soil by organic amendment. Arid Soil Research and Rehabilitation 8:9-17.
L6pez Bermfxdez, F., and J. Albaladejo. 1990. Factores ambientales de la degradaci6n del suelo en el Area mediterr~inea In J. Albaladejo, M. A. Stocking, and E. Dfaz, (eds.), Soil degradation and rehabilitation in Mediterranean environmental conditions. CEBAS-CSIC, Madrid. Murphy, J., a n d J . P. Riley. 1962. A modified single solution method for the determination of phosphate in natural waters. Analytical Chimica Acta 27:31-36. Olsen, S. R., C. V. Cola, F. S. Watanabe, and L. A. Alan. 1954. Estimation of a~ilable phosphorus in soils by extraction with sodium bicarbonate. USDA Circular 939. Pal, D., and F. E. Broadbent. 1975. Influence of moisture on rice straw decomposition in soils. Soil Science Society of America Proceedings 39:59-63. Rad, J. C. 1992. Materia orgfi.nica residual urbana: extracci6n y caracterizaci6n de actividades enzimfi.ticas de inter6s agrotecnol6gico. PhD thesis. Universidad de Valladolid, Colegio Universitario de Burgos, Burgos, Spain. Soil Survey Staff. 1994. Keys to soil taxonomy. USDA. Soil Conservation Service, Washington, DC. Spanish Group of Analytical Method Standardization. 1978. Determinaciones analfticas en suelos. Normalizaci6n de mStodos. Densidad real. Anales de Fdafologia y Agrobiologia 37:1003--1016. Yeomans, J. C., andJ. M. Bremner. 1989. A rapid and precise method for routine determination of organic carbon in soil.
Communications 19:1467-1476.
in
Soil
Science
and
Plant
Analysis