FIRE-FREE FALLOW MANAGEMENT BY MECHANIZED CHOPPING OF BIOMASS FOR SUSTAINABLE AGRICULTURE IN EASTERN AMAZON: EFFECTS ON SOIL COMPACTNESS, POROSITY, AND WATER RETENTION AND AVAILABILITY José Miguel Reichert 1 , Miriam Fernanda Rodrigues 1 *, Clóvis Moisés Priebe Bervald 2 , Osvaldo Ryohei Kato 3 1 Soils Department, Federal University of Santa Maria (UFSM), Santa Maria, RS, Brazil, 97105-900 2 GSI Brazil, Chapada dos Guimarães, MT, Brazil 3 EMBRAPA Eastern Amazon, Belém, PA, Brazil Received 8 October 2014; Revised 30 April 2015; Accepted 1 May 2015 ABSTRACT Many Amazon soils are naturally fragile and develop under intense rainfall and high temperature climate. Soil structural quality is improvable through vegetation management systems providing abundant soil mulch and little soil disturbance. We tested the effect of the chopping- and-mulching systems of secondary forest-vegetation on selected soil physical properties of a sandy Oxisol, located in the eastern Brazilian Amazon. The treatments consisted of two fallow forest-vegetation, namely, chop-and-mulch of secondary vegetation with a dual vertical- rotor chopper (VC) and with a horizontal-rotor chopper (HC) and a control composed of standing secondary forest-vegetation (SF). Six months after vegetation chopping, we evaluated soil bulk density (BD), degree-of-compactness (DC), pore-size distribution and soil hydraulic prop- erties. Compared with SF, the VC and HC systems did not affect soil BD in surface layer (0·00–0·05 m) but increased BD in subsurface layers. DC in surface layer are below the range of the optimum values for most crops (77%–88%), while for other layers DC was within this optimum range. High macroporosity and sandy granulometry were responsible for high drainable water from soil saturation until field capacity, down to 0·20 m depth in all systems. Pore-size distribution was influenced by vegetation chopping with increase in smaller pores. These pores represent a small percentage of total porosity and contribute more to water retention and availability. Chopping by VC and HC mechanisms was effective in maintaining soil physical quality, characterized by lower soil bulk density, higher porosity and water retention, build up during growth of secondary vegetation in fallow period between cash crops. Copyright © 2015 John Wiley & Sons, Ltd. key words: mechanized biomass chopping; forest mulch; soil physical properties; secondary humid forest vegetation; sustainable agriculture INTRODUCTION Soil management traditionally adopted by eastern Amazon farmers is mostly slash-and-burning of secondary vegetation developed during fallow period (Denich et al., 2005). Al- though some studies do not clearly show negative effect on soil caused by fire management (Doerr & Cerdà 2005; Cerdà & Doerr 2008; Mataix-Solera et al., 2011; Guénon et al., 2013; Wang et al., 2015), slash-and-burning generally increases organic matter decomposition rate and clay disper- sion, thus reducing aggregate stability and increasing soil bulk density of subsurface layers (Braz et al., 2013) and in- creases soil surface temperature (Ferreira et al., 2002) and evaporation (Cochrane & Sanchez 1982) because of the in- crease of incident solar radiation on the soil surface. In slashing-and-burning agriculture, crop yield declines over time (Kanashiro & Denich 1998) and farmers are obliged to abandon agriculture fields and farm adjacent fields. This shifting agriculture is not sustainable (Hölscher et al., 1997), particularly in fragile soils. Fire effects on soil are long-lasting (Cerdà & Lasanta 2005; Lasanta & Cerdà 2005). Land managements with the use of chipped biomass are essential for soil protection of road embankments (Lee et al., 2013), constructed soil in coal mines (Milder et al., 2012; Mukhopadhyay & Maiti 2014), afforested land (Jiménez et al., 2013) and agriculture land (García-Orenes et al., 2012; Tejada & Benítez 2014). Chop-and-mulching biomass from forest regrowth is an alternative for traditional slash-and-burn management (Kato et al., 2002; Comte et al., 2012; Reichert et al., 2014; Reichert et al., 2015), as demonstrated in a large-scale pro- ject of Embrapa Eastern Amazon, in cooperation with the German Government, in Igarapé-Açu, Pará State, Brazil, called Tipitamba or SHIFT Studies of Human Impact on Forests and Floodplains of the Tropics (Kanashiro & Denich 1998; Comte et al., 2012). In chop-and-mulching management system, forest-vegetation biomass build-up during fallow periods is the main source of organic matter and nutrients for crops (Kato et al., 2002). Main- taining chopped residue on soil surface prevents nutrient loss; reduces environmental pollution, fire hazards and exposure to soil erosion agents (Ewel et al., 1991; Kato et al., 2002; Braz et al., 2013); provides benefits to the ecosystem; and in- creases crop productivity (Denich et al., 1998; Hughes et al., 2000; Denich et al., 2004; Sommer et al., 2004). Further, there * Correspondence to: M. F. Rodrigues, Soils Department, Federal Univer- sity of Santa Maria (UFSM), Santa Maria, RS, Brazil, 97105-900. E-mail: miriamf_rodrigues@yahoo.com.br Copyright © 2015 John Wiley & Sons, Ltd. land degradation & development Land Degrad. Develop. 27: 1403–1412 (2016) Published online 29 May 2015 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/ldr.2395