The potential use of two restoration strategies to activate biogeochemical nutrient cycles in degraded soils in Colombia was studied. The active model was represented by forest plantations of neem (
Land degradation in arid and semiarid lands increases as a result of soil misuse or mismanagement, which, together with climatic variations, may promote desertification and reduces soil productivity [
The hypothesis of this study is that the activation of soil biogeochemical nutrient cycles and soil quality improvement of degraded dry land depend on the strategy of restoration (active and passive). Thus, the objective of this study was to evaluate the potential use of both active and passive strategies to restore soil biogeochemical nutrient cycles in fine litterfall and soil quality in tropical degraded dry lands by overgrazing. The active restoration strategy consisted of a plantation of neem (
This study was conducted in Santa Fe de Antioquia, northeastern Colombia (6°54′N, 75°81′W, 560 m of altitude). The annual average temperature, sunlight, precipitation, and evaporation of the region are 26.6°C, 2172 h, 1034 mm, and 1637 mm, respectively, characterized by a pronounced annual water deficit derived from the Precipitation/Evaporation ratio of 0.63. The landscape consists of hills with a low to medium slope formed from sediments from the Tertiary. The soils are alkaline and classified as Typic Ustorthents (USDA soil taxonomy); the most dominant soil use is grassland and unfortunately it is degraded by overgrazing. The neem plantations studied here (active strategy) were established in 2004 on hillsides severely eroded by overgrazing; since then, forestry management practices have not been carried out. In the middle of these plantations have grown natural successional patches (passive strategy), which are constituted by native plant species heavily dominated by mosquero.
To evaluate the biogeochemical cycle processes we established 20 circular plots of 250-m2 in forest plantations (FPN) and 13 similar plots in successional patches (SPM) (Table
Mean values of structural parameters of the successional patches of mosquero (SPM) and forest plantations of neem (FPN) studied in Antioquia (Colombia); standard deviation in parentheses.
Parameters | FPN | SPM |
---|---|---|
DBH (cm) | 3.55 (0.85) | 1.88 (0.97) |
|
3.89 (0.92) | 2.91 (0.83) |
|
3.81 (0.46) | 1.84 (0.53) |
|
1.36 (0.76) | 0.76 (0.24) |
DBH: diameter at breast height (1.3 m),
The litterfall was separated in the following fractions: (a) neem leaves (NL), (b) mosquero leaves (ML), (c) leaves from other species (OL), (d) wood material (WM) from branches of <2-cm diameter and small bark pieces, (e) reproductive material (RM), and (f) other materials (OM). The divided material was oven dried at 65°C and weighed. The samples of NL and ML were then separately combined and homogenized for every two 15-day periods (1 month) and a subsample was taken for chemical analysis. At the end of the year, samples from the accumulated litterfall layer or standing litter layer on the soil of the plantations and successional patches (a
The decomposition of neem and mosquero leaf litter was studied by installing 18 litter-bags (
Leaf nutrient contents were analyzed by different methods: carbon (C) by the Walkley and Black method, nitrogen (N) by the Kjeldahl method, phosphorus (P) by the molybdate-blue method; calcium (Ca), magnesium (Mg), and potassium (K) by atomic absorption spectroscopy.
In soil samples the methods used were pH (1 : 2, water), total N (Nt) (Kjeldahl method), available P (Bray-Kurtz’s method), exchangeable Ca, Mg, and K (1 M ammonium acetate; atomic absorption spectroscopy). We also determined soil bulk density (BD) and aggregate stability (AS) (Yoder method). Details about soil and plant analysis methods are available in Westerman [
The rate of potential nutrient return (PNR) by the leaf litterfall was calculated as the product of the nutrient concentration and the dry mass of the leaf litterfall. The retention of nutrients in the standing litter (RNSL) was obtained by multiplying the dry weight of the leaves present in the standing litter and their respective concentration. We calculated nutrient release from the decomposition coefficient
The weight loss in the litter-bags was expressed by a simple exponential model [
Regression models were fitted using nonlinear regression for the weight loss of leaf material deposited in the litter-bags. Linear coefficient of determination (
The results of this study clearly showed that there were significant differences between the active and passive strategies characterized here to activate soil biogeochemical nutrient cycles in tropical dry lands. The annual production of leaf litter and total fine litter per unit area was 2.6-times and 1.6-times higher in SPM than in FPN (Table
Mean values of fractions for fine litter production (FLP) (kg ha−1 yr−1) and standing litter (SL) (kg ha−1) in successional patches (SP) and forest plantations (FP). Standard deviation is in parentheses. ML and NL: leaf litter of mosquero and neem in their respective ecosystem; OL: other leaves; RM: reproductive material; WM: woody material; OR: other rests unidentified.
Fraction | FLP | SL | ||||||
---|---|---|---|---|---|---|---|---|
(kg ha−1 per two weeks) | (kg ha−1 yr−1) | (kg ha−1 yr−1) | ||||||
SPM | FPN |
|
SPM | FPN | SPM | FPN |
|
|
ML, NL | 15.4 (10.6) | 8.0 (6.2) | 2.85** | 477.9 | 184.8 | 238.9 | 73.1 | 4.87*** |
OL | 3.1 (3.1) | 7.3 (3.3) | 4.43*** | 116.9 | 176.9 | 46.7 | 181.3 | 2.35 |
RM | 8.7 (6.9) | 5.8 (6.0) | 1.07 | 227.1 | 135.8 | ND | ND | ND |
WM | 1.7 (1.6) | 1.9 (0.9) | 0.74 | 52.7 | 48.1 | 64.3 | 111.8 | 0.17 |
OR | 0.5 (1.0) | 0.5 (0.5) | 0.28 | 27.6 | 11.9 | 19.11 | 84.9 | 0.0002 |
| ||||||||
Total | 29.3 (18.3) | 23.4 (11.5) | 1.09 | 902.2 | 557.5 | 369.0 | 451.1 | 1.08 |
**, *** Denote significant differences between means at
ND: not determined.
In the results we saw a contradiction about which type of leaf litter decays faster. The constant of decomposition (
Fitted models for residual dry matter (RDM) as a function of time for neem and mosquero leaf litter.
Plant species | Model |
|
|
|
|
SSR | D-W |
---|---|---|---|---|---|---|---|
Mosquero ( |
RDM = 1.29 * |
0.21 | 1.37 | 3.36 | 0.97 | 0.06 | 1.31 |
Neem ( |
RDM = 3 * |
0.44 | 2.91 | 1.58 | 0.84 | 0.01 | 0.18 |
Residual dry matter (RDM) from leaf litter of neem (
The high rate of decomposition of organic debris in SPM and therefore their low residence time are aspects of special significance to the reactivation of biogeochemical nutrient cycle in these degraded soils [
In both SPM and FPN, nutrient concentrations in the leaf litter (LLC) followed the sequence
Mean monthly values (±SD) for leaf litter nutrient concentration (LLC, %) and potential nutrient return (PNR, kg ha−1) via leaf litter in successional patches of mosquero (SPM) and forest plantations of neem (FPN) studied. Coefficient of variation (%) in parentheses.
Nutrienta | LLC (%) | PNR (kg ha−1) |
|
|
| ||
---|---|---|---|---|---|---|---|
SPM | FPN | SPM | FPN | ||||
N | 1.08 ± 0.13 |
1.29 ± 0.31 |
0.44 ± 0.35 |
0.20 ± 0.18 |
2.105 | 0.047* | 12 |
P | 0.05 ± 0.01 |
0.03 ± 0.01 |
0.02 ± 0.02 |
0.005 ± 0.004 |
3.052 | 0.006** | 12 |
Ca | 1.75 ± 0.33 |
2.16 ± 0.65 |
0.76 ± 0.60 |
0.39 ± 0.44 |
1.721 | 0.099NS | 12 |
Mg | 0.59 ± 0.08 |
0.46 ± 0.06 |
0.25 ± 0.21 |
0.07 ± 0.06 |
2.829 | 0.010** | 12 |
K | 0.28 ± 0.13 |
0.29 ± 0.14 |
0.09 ± 0.08 |
0.04 ± 0.03 |
2.226 | 0.036* | 12 |
The potential nutrient return (PNR) through leaf litter followed the same sequence of concentrations (Table
The real nutrient return (RNR) via leaf litter was higher in SPM for all nutrients (Table
Indexes calculated for return, retention, and release of nutrients via leaf litter in successional patches of mosquero (SPM) and forest plantations of neem (FPN) (kg ha−1 yr−1).
Indexes | SPM | FPN | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
C | P | Ca | Mg | K | N | C | P | Ca | Mg | K | N | |
PNR | 114.4 | 0.22 | 8.4 | 2.8 | 1.3 | 5.2 | 45.9 | 0.06 | 4.6 | 0.9 | 0.5 | 2.4 |
RNSL | 67.9 | 0.18 | 1.6 | 0.2 | 0.9 | 3.9 | 18.0 | 0.03 | 1.5 | 0.3 | 0.1 | 1.8 |
|
0.6 | 0.55 | 0.8 | 0.9 | 0.6 | 0.57 | 0.7 | 0.68 | 0.7 | 0.7 | 0.9 | 0.6 |
MRT | 1.6 | 1.81 | 1.2 | 1.1 | 1.6 | 1.76 | 1.4 | 1.47 | 1.3 | 1.3 | 1.1 | 1.8 |
RNR | 71.8 | 0.12 | 7.04 | 2.6 | 0.8 | 2.94 | 33.0 | 0.04 | 3.5 | 0.6 | 0.5 | 1.4 |
PNR: potential nutrient return rate (kg ha−1 yr−1), RNSL: retention of nutrients in the standing litter (kg ha−1 yr−1),
Mean values (±SD) for some soil parameters (0–10 cm) in successional patches of mosquero (SPM), forest plantations of neem (FPN), and control sites without vegetation studied in Santafe de Antioquia (Colombia).
Parametera | Control sites | SPM | PCI | FPN | PCI |
---|---|---|---|---|---|
pH |
|
|
0.99 |
|
1.01 |
SOM (%) |
|
|
2.19 |
|
1.72 |
Nt (%) |
|
|
1.19 |
|
1.27 |
P (mg kg−1) |
|
|
0.53 |
|
1.31 |
Ca (cmolc kg−1) |
|
|
1.87 |
|
1.15 |
Mg (cmolc kg−1) |
|
|
2.11 |
|
1.10 |
K (cmolc kg−1) |
|
|
1.14 |
|
1.61 |
ECEC (cmolc kg−1) |
|
|
1.98 |
|
1.14 |
BD (Mg m−3) |
|
|
0.93 |
|
0.93 |
AE (%) |
|
|
0.94 |
|
1.10 |
PCI: parameter change index (FPN/control or SPM/control), SOM: soil organic matter, ECEC: effective cation exchange capacity, BD: bulk density, AS: aggregate stability. *Indicates significant difference with control sites (Mann-Whitney,
In terms of the return and incorporation of organic matter and C into the soil by leaf decomposition in the SL, the superiority of SPM was notorious (Table
N and P were released more slowly (lower values of
Soils of both SPM and FPN showed changes of some properties with respect to soil of control sites (without vegetation) (Table
Despite their short period of time for both strategies, the contributions of fine litter and its decomposition have improved various soil properties of these degraded lands. The sharp increases of SOM observed (compared to control sites) also increased soil moisture retention capacity and soil cation exchange, key aspects in the reclamation of soils of degraded dry land. Although FPN showed a significant increase of P, its very low concentration in the soil determined a severe constraint on ecosystem primary productivity.
From the perspective of land restoration, both models showed different advantages. The passive model represented by the SPM showed a higher dynamics in the reactivation of soil biogeochemical cycles. It is expected that as the successional process continues the consequently greater complexity of the ecosystem will lead to an effective improvement not only on the soil, but also on ecosystem functions. On the other hand, the active model represented by the FPN showed significant improvements in soil parameters, even though the returns of litter and nutrients were lower. Likely, this situation is the result of differences in litter contributions, whose potential effect on soil rehabilitation has not been fully evaluated. These are issues to consider in selecting a restoration model and the degree and speed expected of the degradation process. Thus, an active model should be considered when the rate of degradation of the area of interest is high, because the planted species can be established quickly and create better conditions for a more diverse biological community as pointed by [
The authors thank the Direction of Research of the Universidad Nacional de Colombia for financial support of the Project “Restoration of lands in a process of desertification with neem plantations (